In recent years, major advances have been achieved in the understanding of pulmonary arterial hypertension (PAH) patho-physiology. Associated pulmonary arterial hypertension (APAH) can occur in a variety of other conditions and circumstances including a number of systemic autoimmune diseases. As with PAH in general, clinical symptoms of APAH in systemic autoimmune diseases are unspecific. In addition, there is a long standing association between autoimmunity and APAH. It has been postulated that autoimmunity may play a role in the pathogenesis of APAH. This argument has been based on frequent coexisting clinical and serological rheumatic findings. There is no experimental model of immune mechanism-dependent severe APAH. The loss of self-tolerance could initiate a process which ultimately results in APAH. It is possible that T-cell deficiencies (in either function or number) may contribute to pulmonary vascular injury or disease. These conditions are often associated with autoantibodies as well as defects in the CD4 T-cell compartiment. However, it remains uncertain how autoimmune mechanisms contribute to the pathogenesis of APAH. There are data that show a significant association between APAH and connective tissue diseases (CTD). In this regard, systemic sclerosis, mixed connective tissue disease, systemic lupus erythematosus, dermato/polymyositis and primary Sjögren's syndrome are associated with APAH. The study of APAH in the systemic autoimmune diseases and its relation to basic immunologic disturbances may yet bring effective therapies in the future. APAH can be a severe complication attracting a high excess mortality in autoimmune diseases. The present review will focus on what is known about autoimmune phenomena in APAH patients.