Five cases of psoriasis pustulosa were examined by electron microscopy. The main features within the dermis were dilated capillaries filled with polymorphonuclear leukocytes and red cells. Neutrophils passed out of the vessels through the gaps and fenestrations and migrated towards the epidermis throughout the distinctly edematous corium. Lymphatic vessels could be found. Histiocytes containing Langerhans granules were observed in the dermis and epidermis. The morphological changes of the epidermis depented on the place examined and were most evident near fully developed pustules in the upper layers. The presence of previously described morphological findings could be confirmed. Perinuclear cytolysis and other changes in the keratinocytes, however, seem to be secondary, resulting from edema and accumulation of neutrophils within the epidermis. Immunological phenomenona may be responsible for the latter event.