Rough skin ("dry skin") is characteristic of atopy and thought to be a pre-stage of atopic eczema. Atopic eczema frequently deteriorates in winter, which may be related to low air humidity during the heating period. We have assessed skin roughness before and after decreasing air humidity to 30% in a climatic chamber. Replicas of clinically non-inflamed and not scaling skin from 10 patients with atopic eczema and 10 controls were taken before and after lowering air humidity for 3 h and were analyzed for the roughness parameters Ra, RZ(DIN) and RZ(ISO). After exposure to low air humidity there was a significant increase of RZ(DIN) from 61.5 microns to 66.9 microns (p < 0.05) and of RZ(ISO) from 63.8 microns to 66.4 microns (p < 0.05) in patients with atopic eczema, whereas no significant change occurred in controls. Development of skin roughness over 3 h under natural indoor environmental conditions did not indicate spontaneous variations. These quantitative data show that a short period of exposure to low air humidity increases skin roughness and may particularly influence the condition of patients with atopic eczema.