To establish the character of microvascular changes in psoriatic skin and their specificity, 29 skin biopsies of psoriatics (20 in exacerbation and 9 in a stationary stage) were investigated using histologic, histochemical, immunomorphologic, electron-microscopic, and morphometric methods. Five biopsies of uninvolved skin in scleroderma and five of diabetes mellitus patients were studied with the same technique for comparison. The results showed that structural changes depend on disease stage and the clinical appearance of lesions. Microvascular changes precede papule appearance during exacerbation and gradually increase during papule development. They comprise vascular dilatation, bridged fenestrations and gaps in endothelium, edematous areas in the cytoplasm of endotheliocytes, myocytes and pericytes, basement-membrane-zone thickening, and cell extravasation--signs of increased vascular permeability. Immunoglobulin G deposits in vascular walls, degranulation of mast cells, and extravasation of lymphocytes and neutrophils indicate that inflammation is a basic process during exacerbation and that immune mechanisms play an important role in the pathogenesis of inflammation. Microvascular changes in scleroderma and diabetes mellitus are different in nature and do not resemble those in psoriasis.