We report on 2 patients with well-documented mixed connective tissue disease (MCTD) accompanied by severe pulmonary hypertension (PH) due to thrombosis or thromboembolism. In a previous report we indicated (1) that patients with MCTD complicated by PH have a significantly worse prognosis than patients with other connective tissue disease (CTD) complicated by PH. Both our patients had anticardiolipin antibody (a-CL) in the initial stages of the disease. We also studied the relationship of a-CL to PH in patients with other CTD. Patients of either MCTD or SLE with high levels of a-CL had significantly higher values of mean pulmonary arterial pressure than patients without a-CL. Several factors were suggested for the pathogenesis of PH such as vasospasm, arteritis, platelet dysfunction, and thrombosis or thromboembolism. The presence of a-CL may be one of important factors in development of PH among patients with MCTD with recurrent pulmonary thrombosis or thromboembolism.