The clinical and serological findings on 24 patients with pulmonary hypertension (PHT) seen at the Lupus Clinic of St. Thomas' Hospital, London are presented. Twenty-two patients had systemic lupus erythematosus (SLE), one other a primary antiphospholipid syndrome and another an SLE/progressive systemic sclerosis (PSS) overlap syndrome. In 21 of the 24 patients, the disease resembled the primary idiopathic variety with clear lung fields and no clinical evidence of pulmonary thromboembolism, although angiography and nuclear perfusion scans were not performed. Two patients clearly suffered from thromboembolic PHT, one with SLE and one with an antiphospholipid syndrome. One patient with SLE/PSS overlap syndrome developed pulmonary fibrosis. The frequency of antiphospholipid antibodies (lupus anticoagulant and antibodies to cardiolipin was 68% which appears to be higher than generally found in patients with SLE, and the clinical significance of this finding is unknown. Other associated features of the antiphospholipid syndrome in this group were uncommon. Death occurred in 13 of the 24 patients, 4 were lost to followup and 7 are known to be alive. The cause of death was circulatory failure in the majority; sudden death once this complication occurred was particularly common. One patient died from adult respiratory distress syndrome and one from hemorrhagic shock while undergoing heart/lung transplantation. Two patients underwent successful heart/lung transplantation. One, however, died of a mesenteric occlusion and bowel infarction following a second lung transplantation because of rejection of the first heart/lung transplantation after one year. The other patient is alive and well 2 years later.