Light and electron microscopic studies were performed on the synovial membranes from the large joints of 12 patients with psoriatic arthritis. The most significant histopathologic findings were vascular changes that were characterized by endothelial cell swelling accompanied by inflammatory cell infiltration and marked thickening of the vessel wall. Fibrosis of the subsynoviocyte tissue was also prominent, especially in patients with chronic disease. Villi were not prominent, and the lining layer was not thickened. Both type A and B cells were seen; none had significant abnormalities. No electrondense deposits were observed. These findings lend further support to the theory that microvascular changes play a significant role in the pathogenesis of psoriasis and its related arthritis.