In former studies we have shown the increased angiogenic capability of peripheral blood mononuclear cells (MNCs) from patients with the active guttate type of psoriasis vulgaris. The present study tested the effects of serum factor from 67 patients with various forms of psoriasis vulgaris and from 32 healthy individuals on the angiogenic capability of normal human MNCs in an animal system. The angiogenesis-enhancing effect was most marked in the serum samples from patients with plaquelike and peripherally spreading lesions lasting for one to two months, and tended to disappear in patients with long-lasting and/or stationary lesions. Serum samples from patients with psoriasis were also capable of modulating the proliferation of normal human endothelial cells in vitro, and this modulation was correlated with the duration of relapse. The effects of psoriatic sera on normal human MNC function and on endothelial cell growth may be of importance for vascular proliferation in psoriasis.