Although it is well known that the skin in patients with atopic dermatitis becomes drier in winter, the mechanisms of winter deterioration of dry skin are not fully understood. Our purpose was to determine whether residual washing detergent in cotton clothes plays a role in the winter deterioration of atopic dry skin. We studied 148 Japanese patients with atopic dermatitis who visited our dermatology clinic during winter months. They wore cotton underwear, which they had washed in cold tap water. We examined the distribution of dry skin on their trunks. We then asked them to stop washing their clothes with common anionic, additive-enriched detergents, and to use a nonionic, additive-reduced detergent for a period of two weeks. Photographs of 2 or 3 representative dry skin sites on the trunk were taken before and after the trial. By comparing the before-after trial photographs, the severity of dry skin at the end of the trial was assessed on a 5-point scale ranging from markedly improved to worsened. Of the 148 patients examined, 115 (78%) had widespread or localized dry skin on the trunk. The dryness of the skin was prominent around the shoulders. Of these 115 patients, 87 (76%) showed marked or moderate improvement of dry skin after the two-weeks of use of the nonionic, additive-reduced washing detergent. No patient showed worsening of the dry skin. These results suggest that residues of common washing detergents in cotton underclothes play an important role in the winter deterioration of dry skin in patients with atopic dermatitis who use cold tap water for washing their clothes.