Pulmonary arterial involvement is an important complication of Behçet's syndrome (BS). Among 2179 patients with BS, 24 (1.1%) were diagnosed as having pulmonary arterial aneurysms (PAAs). Haemoptysis was the presenting symptom in all but one. All were male. The mean age at the time of the diagnosis of PAA was 30 +/- 11 S.D. yr (range 17-59 yr). Their mean disease duration was 5 +/- 4 yr (range 3 months-16 yr). There was a high prevalence of thrombophlebitis (21/24, 88%). Histopathological examination showed pulmonary vasculitis involving all layers of pulmonary arteries and veins. Twelve patients (50%) died after a mean of 9.5 +/- 11 S.D. months (range 1-36 months) after the onset of haemoptysis. The mean duration of follow-up of the remaining 12 patients was 25.5 +/- 24 S.D. months (range 1-78 months). The treatment consisted mainly of pulsed or oral cyclophosphamide alone or with prednisolone. As is true with other severe manifestations of Behçet's syndrome, PAAs are more common among males. They are associated with a prevalence of thrombophlebitis and there is high mortality despite treatment.