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  • Lightweight, breathable, pure silk jersey is enhanced with a wicking treatment to help keep you dry and warm; plus, they dry quickly
  • Wide, self-fabric elastic waist, seamless sides and rib-knit cuffs provide a comfortable fit throughout
  • Fabric is substantial yet thin enough for everyday wear under jeans or casual pants
  • Machine wash warm on gentle cycle without bleach or fabric softeners; tumble dry low


View all REI Co-op Men's Long Underwear Bottoms

REI membership


Best Use Multisport
Fabric Silk
Moisture Wicking Yes
Quick Drying Yes
Underwear Weight Lightweight
Inseam (in.) 30.5 inches
Gender Men's

Size chart

Sizing notes

SHORT: Some pants are available in short sizes, which generally means an inseam that is 2.5 inches shorter than regular sizes. TALL: Some pants, shirts, jackets and parkas are available in tall sizes, which generally means an inseam that is 3 inches longer and a rise that is 1 inch longer, a shirt or jacket body that is 2 inches longer, a parka body that is 2.5 inches longer and sleeves that are 2.5 inches longer than regular sizes.

Rated 3 out of 5 by from Warm & Light but Questional Durability I ordered these online and after receiving them was shocked to see how thin they are. They really are a light weight garment but somehow manage to be warm. Granted, I used them for spring skiing but they are much warmer than you'd think by appearance alone. So the good: lightweight & warm The bad: The fly is too high for "easy access"; it takes a bit of work to use the restroom. Second, I'm concerned about long term durability of these. I was taking off my ski pants and the edge of my suspenders caught on the fabric, leaving a tear similar to what you would see in pantyhose. I'm going to keep an eye on that hole next year and if it grows I plan to return these. Overall I still recommend, but not without reservation.
Date published: 2013-03-25
Rated 4 out of 5 by from This is silk, not burlap. Many of the comments about silk products center around durability. Well guys, it's silk we're talking about here, so let's just keep that in mind. They're going to blow out at the elbows for sure, and probably the knees too, after a season or two or three depending on your level of usage. These are not meant to be family heirlooms. But they are a little slice of heaven on earth, and also make for the best pajamas ever. When mine blow out I cut them off at the elbows or knees and continue to use them as base layer (after layer after layer after layer) and extend their useful life for years. I've been doing this for literally decades. That way they're still regulating your core temperature, which we all know is the key. So just be realistic and enjoy these wonderful garments for what they are! Note: they are fail-safe Christmas presents!!!
Date published: 2013-11-15
Rated 5 out of 5 by from Comfortable under all conditions The good thing about these is the comfortable fit. They can easily be worn under any clothes. And going indoors does not lead to feeling overly warm. That's the great thing about silk. Earlier models felt a bit smoother still, but these are fine as well, and probably a bit more durable. I tend to have problems finding the opening, thus need to "grope" more than I like. That's the only minor complaint. Paid a lot of postage to have them sent overseas, as I knew these were what I wanted again.
Date published: 2012-02-16
Rated 5 out of 5 by from Outstanding Product Been wearing these for many years. The product was slightly better a few years ago with multiple colors but remains outstanding. Best part is they provide great warmth when outside but are comfortable inside all day long. Nothing like silk. They wear well and even with constant use about five months a year and daily washing they hold up for two or three years.
Date published: 2011-11-19
Rated 4 out of 5 by from Good for the cold Recent CA transplant to MD. This is the perfect match for the colder climate. Very light and comfortable. Probably too light for extreme cold.
Date published: 2013-01-17
Rated 5 out of 5 by from Great for the cold I use them everyday that I need to work outdoors. I place them in my backpack with my work clothes and use them regularly. Unexpectedly durable, easy to pack and extremely warm when they need to be. recommended these to family and friends.
Date published: 2013-02-07
Rated 2 out of 5 by from very comfy but substandard quality I've been using silk long undies for decades during the winter. By hand washing, I'd usually get about 3 winters' use out of a pair. Finally gave a couple pairs of the REI brand a try at the beginning of last winter. Now that it's getting cold again they're back in rotation, but the weave on the REI silks is coming undone in about 5 random places. One season durability is not a good deal.
Date published: 2013-10-14
Rated 4 out of 5 by from Good for sleeping My legs tend to get a bit sweaty when I'm sleeping, and these take care of that problem. It seems like I'm also sleeping warmer, but haven't used them in anything lower than mid/upper-30s. They're super light and packable, which is a plus.
Date published: 2013-06-27
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Do they have an opening in front?

Asked by: Rucio
Yes, they do have an opening in the front.
Answered by: REIservice
Date published: 2015-09-20

1) how much does this product weigh? 2) repeat question for all your products. dam!

Asked by: Steve Flinn
The silk bottoms weigh 3.2 ounces.
Answered by: REIservice
Date published: 2017-02-02

Do they have a seamless "butt" panel?  Or a seam in the middle of the back?

Asked by: Velera
​There is a seam in the middle of the back and front of these bottoms.
Answered by: REIservice
Date published: 2017-03-30

This is not as much of a question as it is advise, but I do agree with Steve Finn (although he could have put it in a more mature way), knowing the overall weight of the items you have (especially for backpacking, hiking, etc.) can be really helpful.

Asked by: Xx_CampingMan_xX
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