152 patients with thromboangiitis obliterans (Buerger's disease) and pain from critical leg ischaemia were randomly allocated to receive iloprost, a chemically stable prostacyclin analogue, or low-dose aspirin, for 28 days in a double-blind trial. On review, 19 patients did not fulfil the stringent entry criteria. Of the other 133 patients, 98 also had leg ulcers. After 21-28 days, 58 (85%) of 68 iloprost-treated patients showed ulcer healing or relief of ischaemic pain, compared with 11 (17%) of 65 in the aspirin-treated group. 43 (63%) on iloprost treatment had complete relief of pain, compared with 18 (28%) on aspirin. Ulcers healed completely in 18 of 52 (35%) who received iloprost compared with 6 of 46 (13%) who received aspirin. 6 months after the start of treatment, the response rate was 45 of 51 (88%) patients treated with iloprost compared with 12 of 44 (21%) patients treated with aspirin.