Antiphospholipid antibodies have been associated with two types of pulmonary hypertension (PHT), the thromboembolic type, after deep venous thromboses in the lower limbs complicated by pulmonary embolism and the "primary" plexogenic type. The PHT may occur in the absence of any other manifestations of the antiphospholipid syndrome (APS), and cases have been recorded with very high levels of antiphospholipid antibodies. It may also accompany systemic lupus erythematosus (SLE) and may manifest with or without other features of the APS. It may also form part of the clinical presentation of a "primary" APS. Its prevalence is of the order of 1.8-3.5% of the manifestations of the APS depending on the series. Primary "idiopathic" PHT has long been regarded as an "immunological" disorder. Its manifestations are essentially to the primary type seen with the connective tissue disorders such as SLE, APS, mixed connective tissue disease, calcinosis, Raynaud phenomenon, esophageal dysmotility, sclerodactyly, and telangiectasia variety of systemic sclerosis and Sjögren's syndrome. The high prevalence of PHT in patients with human immunodeficiency virus infection who demonstrate low CD4 counts points to a close relationship between the T regulatory cells (Treg) and the development of PHT, and this hypothesis is discussed in this review. Genetic and chromosomal aspects of PHT are also discussed.