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  • Fine merino wool helps maintain a comfortable temperature whether the day is hot, cold or in between
  • Chafe-free flatlock seams enhance comfort
  • Fabric protects skin from harsh UV light with a UPF rating of 30
  • Wide waistband enhances fit
  • Functional fly
  • Unlike traditional wool, Smartwool items do not itch and can be repeatedly washed and dried without shrinking
  • The formfitting Smartwool Microweight long underwear bottoms for men offer a minimalist fit and a natural rise


View all Smartwool Men's Long Underwear Bottoms

REI membership


Best Use Multisport
Fabric Merino wool
Sun-Protective Fabric Yes
UPF Rating 30
Moisture Wicking Yes
Underwear Weight Lightweight
Inseam (in.) 29 inches
Gender Men's

Size chart

Rated 5 out of 5 by from Warm and comfortable I'm a believer in Merino wool products -- they seem to only get softer with wear and washing. Cotton underwear now seems heavy/scratchy to me by comparison. I needed a lightweight thermal bottom for a recent backpacking trip to Alaska and was pleased with the microweight fabric. I wouldn't recommend it for below freezing temps, but for cool nights into the 30's, it provided ample warmth. I love SmartWool socks, and this underwear has a similar build quality.
Date published: 2013-10-24
Rated 2 out of 5 by from Warm, but too thin I wore these on one 9 day elk hunt in the Wyoming mountains and on the 4th day I simply knelt down to start a fire and the right knee tore a large hole about 5" in diameter so the last 5 days I was completely warm except for my freezing kneecap. They worked great otherwise but the material is just too thin to stand up for their intended use. And I was wearing khul pants, which are pretty thick so it wasn't due to abrasion, simply couldn't withstand the stretch of a bending leg. I'll be sending them back for the midweights.
Date published: 2015-11-28
Rated 3 out of 5 by from Amazing product, not durable I love smartwool. It works amazing. However, the microweight isn't very durable, due to its lack of thickness. I wore it on my last snowshoe of the season last year and discovered that the right thigh was perforated with many tiny holes and tears from where it had apparently failed during the wash. Talk about disappointing. I won't get any merino wool product this thin ever again.
Date published: 2014-12-29
Rated 1 out of 5 by from Only Lasts One Season The product is super soft and comfortable, perfect for cooler days when you want to be active. But within 2-3 months of regular use they had already developed big holes that were not repairable. At this price point they are simply too expensive for one season use. Going forward I’ll stick with the mid-weights.
Date published: 2016-01-02
Rated 5 out of 5 by from great for cool weather The Smart Wool microweight merino wool base layer bottoms are great for cooler days (on the Texas Gulf Coast / Piney Woods). They fit true to size
Date published: 2014-02-17
Rated 5 out of 5 by from Keeps me warm at my cold office These keep me warm at the office and out and about without being too warm. They regulate temperature just right, I am never too warm. Nicely designed and fit really well.
Date published: 2016-01-09
Rated 3 out of 5 by from Comfortable I bought these to hike in colder weather. I also used them on a snow ski trip recently where temps were in single digits. They are extremely comfortable, but did not feel they kept me that warm. Hiking and walking, they fit good, just not the warmth I was hoping for. For the money I spent, a bit disappointed
Date published: 2017-01-11
Rated 4 out of 5 by from Almost perfect I like them a lot, smooth outside, fuzzy inside, good quality but I had to return them because the top was too short in the back. I need pants like this a bid longer there, these felt like front was higher than back although I bought size XL and I usually wear M. Never used them.
Date published: 2017-02-07
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What is the exact fiber content of the SmartWool?

Asked by: Rock'n Craid
The tag on the Microweight Long Underwear Bottoms state they are made from 100% merino wool.
Answered by: REIservice
Date published: 2017-01-06
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