Studies were performed in 89 patients with active psoriasis and in 10 normal volunteers. Areas of uninvolved skin of 10 x 10 cm in size were demarcated and closely observed for 14 days. In 54 patients with psoriasis, there developed very fine, erythematous papules almost skin level (with no scaling of the surface), which changed in the course of observation into typical pinpoint papules. Histological and histochemical studies of these finest changes, referred to as prepinpoint papules, revealed fairly abundant infiltrates composed in a large part of polymorphonuclears. The cells penetrated into the epidermis forming focally small accumulations in the stratum corneum. The granular layer became atrophied or blurred, the stratum corneum was orthokeratotic and there was no epidermal proliferation, i.e. no psoriatic features. Immunofluorescence studies showed in about 50% of prepinpoint papules in vivo fixed immunoglobulins, and in 20% of them also complement. The studies point to the role of polymorphonuclears in the earliest stages of the development of psoriatic lesions.