Nine patients with polymyositis or dermatomyositis associated with arthritis without antinuclear antibodies have been studied. In eight patients a mildly inflammatory nonerosive arthritis occurred coincident with early manifestations or before the appearance of symptomatic muscle disease. Hands, wrists and knees were prominently involved. This arthritis responded to corticosteroids given for the myositis. One patient had erosive arthritis and periarticular calcifications associated with acute inflammation that seemed to diminish with colchicine therapy. Synovial biopsy specimens showed surface fibrin deposition, focal loss of lining cells and mild inflammation--findings similar to those in scleroderma. By electron microscopy tubuloreticular structures were found in vascular endothelium in synovium, as in other tissues in polymyositis. All patients surprisingly had pulmonary manifestations suggesting the possibility that the condition in this group of patients may represent a distinctive subset of polymyositis.