66 psoriatics, 34 normal adults, 10 normal juveniles, and smaller groups of patients afflicted either with seborrheic dermatitis-rosacea or atopic dermatitis were examined for the threshold of capillary petechia formation on clinically normal forearm skin in response to repeated tape stripping. A definitely lowered petechial threshold was found in psoriatics (mean of 9 vs. 19 tape strips) and also in persons with the seborrheicrosacea complex. Normal juveniles and patients with atopic dermatitis had an increased threshold. The possible relation of these data are pointed out to recent findings of ultrastructural abnormalities of psoriatic papillary capillaries, and an altered capillary response is suggested as a sine qua non precondition for the development of psoriasis. The petechial threshold test is recommended as a simple adjunct diagnostic tool.