In this report we investigate the simultaneous occurrence of psoriasis and atopic dermatitis (AD) as well as the association with infectious skin diseases. Among 29,159 patients hospitalized between 1953 and 1983, 8.5% (2,467 patients) were treated for psoriasis, while 1.6% (470 patients) were hospitalized for AD treatment. On the basis of incidence rates for both diseases, 36 patients (0.14%) with both psoriasis and AD were expected to be seen. However, the two conditions were simultaneously present in 2 patients only. Approximately 30% of the AD patients were suffering from either bacterial or viral infection, while this complication occurred in 6.7% of psoriatics. In addition, among 48 patients hospitalized for eczema herpeticatum 39 were atopics and none was psoriatic. The data demonstrate that the occurrence of psoriasis and AD in one and the same patient is quite rare and this may be related to conflicting immune defense patterns. Thus, increased sensitization against foreign protein together with high susceptibility to cutaneous infection present in AD is in contrast to high phagocyte responsiveness in psoriasis, where concurrent infections are rare.