The levels of soluble CD30 in 79 patients with atopic dermatitis were compared with those found in 54 patients with psoriasis and 36 control individuals (no psoriasis, no atopic dermatitis). In relation to the control group, patients with atopic dermatitis were found to exhibit an increased concentration of sCD30 of at least 1.5-fold (p < 0.001). In addition, sCD30 concentrations were shown to correlate with the severity of the disease as measured by the score index for atopic dermatitis and different stages of disease activity, such as acute, subacute, or chronic forms, and localized or generalized distribution of atopic dermatitis. The application of topical glucocorticoid therapy for a period of 2 weeks resulted in a decrease in the level of sCD30 by 46% in 8 patients, especially in the acute, generalized form of atopic dermatitis. Psoriasis patients showed no significant differences in sCD30 levels in relation to the control group. This study demonstrates a correlation between sCD30 concentration and the activity of the disease and therefore suggests sCD30 as a prognostic marker, being superior to predictions from measurements of IgE or eosinophil cationic protein.