I did a lot of research before purchasing this jacket and based on trying on Patagonia prior year models of fleece jackets, etc., I was convinced I was an XS. However, the R4 from 2004 fit me so snugly it was very uncomfortable, especially under the arms. I could barely get it off me without much pulling. It had to be returned. I went to a store and found that the small fit me much better. So as others have, I highly advise you purchase one size up from what perhaps you have worn in the past with Patagonia, as the R4 from 2004 and onward runs very small!