Although primary pulmonary hypertension (PPH) is considered to be an idiopathic condition, it has been postulated that autoimmunity may play a role in the pathogenesis of the disease. This argument has been based on frequent coexisting clinical and serological rheumatic findings. Moreover, approximately in a third of the patients with PPH, and antinuclear factor can be detected. Pulmonary hypertension may appear as a secondary complication to various autoimmune conditions. In light of these findings we examined sera derived from 40 patients diagnosed as having PPH for the presence of 18 different autoantibodies by the ELISA and immunofluorescent techniques. Of the 40 patients, 62.4% had circulating autoantibodies and 47.5% presented with multiantibody responses. Autoantibodies most commonly observed were antinuclear (42.5%), anti-ssDNA (25%) and antithyroglobulin (30%) antibodies. These results may imply that in a subgroup of patients with PPH the disease may be ascribed to an immune dysregulation or alternatively that autoantibodies accompany the disease progression as an epiphenomenon.