A retrospective study is presented of all patients with primary pulmonary hypertension (PPH) observed at Zürich University Hospital between 1980 and 1990. In 16 of the 36 patients (23 females/13 males, median age 40) there was a known risk factor for PPH: HIV infection (8), use of anorectic agents (7), liver cirrhosis (1). In 20 cases no risk factor could be found. In 4 of them PPH was diagnosed before the study period. We calculated an incidence of PPH in north-east Switzerland of about 1.5 cases per million per year. The clinical course in the 20 patients without known risk factors for PPH was analyzed in more detail: treatment consisted of oral anticoagulation (15) and vasodilators (7); the median survival was 48 (1-307) months. Only 7 patients (35%) are still alive. Prognosis of PPH correlated with the severity of hemodynamics. The median survival in patients with pulmonary vascular resistance of < 1000 versus > or = 1000 dyn.sec.cm-5 was 151 (106-307) months versus 13 (1-73) months (p < 0.001), with mean pulmonary artery pressure of < 50 versus > or = 50 mm Hg it was 151 (73-307) versus 13 (1-106) months (p = 0.0015), with right atrial pressure of < 10 versus > or = 10 mm Hg it was 90 (4-307) versus 3 (1-60) months, and with mixed venous O2-saturation of > or = 62% versus < 63% it was 119 (4-307) versus 14 (1-106) months (p = 0.024). PPH is a rare disease with a poor prognosis depending mainly on the severity of hemodynamic changes. However, despite this fact, the clinical course of individual patients may be unexpectedly favourable. Treatment of PPH is limited, and lung transplantation remains the most promising therapeutic approach. In this regard the quality of life aspect is of great importance.