Unexplained pulmonary hypertension of nearly simultaneous onset is reported in two brothers, aged 17 and 26 yrs. Echocardiography revealed right ventricular hypertrophy and dilatation, paradoxical septal motion and an enlarged main pulmonary artery. In the right catheterization highly elevated pulmonary arterial pressures (107/58 and 84/46 mmHg) were seen. Doppler echocardiography showed significant leakage of the pulmonary and tricuspid valves in the younger brother, who died suddenly three weeks later. An initial decline in pulmonary arterial systolic pressure was achieved in the older brother by drug therapy. Eight months later, however, the pressure had risen to the pretreatment level. Dyspnoea increased and the patient underwent heart-lung transplantation but subsequently died. In the family study a third brother, father and mother were healthy.