Over a period of up to 18 years, 24 patients (mean age: 33.8 years) with primary vascular pulmonary hypertension (PVPH) of unknown aetiology (group A) and 18 subjects (mean age: 45.3 years) with PVPH due to anorectic drug intake (group B) were comparatively studied. The following main tendencies became apparent: 1) The 10-year cumulative survival rate in group A (0.31) was lower than in group B (0.63). 2) Patients of group A showed more marked X-ray and ECG signs of pulmonary hypertension and right ventricular hypertrophy in comparison with group B. In contrast to group B, the ECG signs of hypertrophy in group A increased during the observation period. 3) Mean pulmonary artery pressure (PAP) significantly increased in group A (from 48.8 to 61.0 mmHg), while it decreased (from 47.6 to 33.3 mmHg) in group B. 4) The diameter of the descending branch of the right pulmonary artery increased with rising PAP only in group A, while the relationship between PAP and the Sokolow-Lyon index was significant only for the whole group of PVPH patients but not for the subgroups A and B. A regression of pulmonary hypertension in patients with anorectic drug intake was obvious, in contrast to the course in patients with PVPH of unknown aetiology.