Chronic pulmonary embolism is a rare disease which can occur at first with pulmonary hypertension. In these cases it may be difficult to distinguish between primary pulmonary hypertension. We examined nine patients with Chronic Pulmonary Embolism (CPE) (three females and six males, mean age 45 +/- 13 years, range 21-67 years) and ten patients with Primary Pulmonary Hypertension (PPH) (seven females and three males, mean age 35 +/- 13 years, range 10-56 years) who came to our attention during the years 1973-1986 (mean follow up 3 years). All patients had an electrocardiogram, chest x-ray, echocardiogram, cardiac catheterization with pulmonary angiography; seven patients with CPE and eight with PPH had perfusion lung scans. Progressive dyspnoea was the main feature in all the patients; four out of nine with CPE and none of the ones with PPH had a previous history of thrombophlebitis. In all the patients the electrocardiogram, chest x-ray and echocardiogram showed signs of pulmonary hypertension, so that a clear distinction between the two groups was not possible. Cardiac catheterization showed pulmonary pressure values higher in patients with PPH as compared to the ones with CPE (systolic pressure 96 mmHg vs 70 mmHg, diastolic pressure 49 mmHg vs 31 mmHg, mean pressure 65 mmHg vs 45 mmHg). Pulmonary angiography in more than half of the patients with CPE showed a "cut off" of two or more lobar branches of the pulmonary arteries. In the patients with PPH pulmonary angiography showed a dilatation of the main pulmonary artery and a diffuse bilateral hypoperfusion. Perfusion lung scan in all the cases of CPE showed zonal perfusion defects, while in all cases of PPH, with the exception of one, it was largely normal. Venograms in the districts of the inferior vena cava demonstrated thrombosis in two out of six patients with CPE. Negative venograms were found in the five patients with PPH who had this investigation performed. One patient with CPE had a surgical embolectomy, the other eight had anticoagulant oral treatment. During the follow-up period three patients with CPE and five with PPH died within five years and within fifteen months respectively, of the diagnosis.(ABSTRACT TRUNCATED AT 400 WORDS)