Levels of soluble interleukin-2 receptors (sIL-2R) were measured as a marker of lymphocyte activation in serum of 37 patients with atopic dermatitis (AD) and of 16 patients with psoriasis vulgaris (PV), all individuals being hospitalized in the Zurich High Mountain Clinic of Davos-Clavadel (altitude 1,600 m above sea level). Measurements were performed at admission and at discharge. The PV patients served as a control group. As a further control, 20 AD patients from the Department of Dermatology in Zurich (altitude 400 m) were also evaluated. The serum sIL-2R concentration in AD patients at admission was significantly elevated in comparison with PV patients and was highly correlated with the severity of the cutaneous involvement. Comparing the 'extrinsic' and 'intrinsic' types of AD, no significant differences of sIL-2R levels were found. The sIL-2R concentrations dropped at discharge. These results suggest that the measurement of the sIL-2R level could be a marker of disease activity.