In the first part of this study peripheral blood lymphocyte subpopulations. Iheir activation slate and various serum parameters were measured in extrinsic and intrinsic atopic dermatitis (AD) patients compared to normal individuals. Beside the characteristic eosinophilia, significantly increased numbers of CD4+ T cells with increased expression of IL-2 receptors (IL-2R) and HLA-DR were noted in the AD patients. In addition, extrinsic AD patients showed increased numbers of CD23+ B cells and decreased numbers of CD16+ natural killer cells. Moreover, increased serum levels of eosinophil canonic protein (ECP) and soluble 1L-2R as well as soluble factors lhat prolong survival of eosinophils in vitro could be demonstrated. In the second section of this study we determine how these blood immunological parameters relate to the clinical severity of the skin lesions of AD, by weekly analysis of 12 AD patients attending a high altitude clinic for 3 to 6 weeks. The patients were divided into two groups on the basis of treatment with topical steroids, but during the observation period a significant improvement in clinical status was observed in all AD patients independent of topical steroid therapy. A progressive decrease in eosinophil and activated T cell numbers. soluble IL-2R levels and serum eosinophil survival prolonging activity could be demonstrated, which closely correlated with the clinical severity of the AD.