Thirty-four patients with ischaemic rest pain in 42 limbs and ankle pressure equal to or less than 50 mmHg have been treated with intravenous infusion of synthetic prostacyclin (iloprost) for eight days. Leg blood flow was measured with air plethysmography before treatment, on day 4 and day 8 of treatment. Total relief of pain for at least 6 weeks occurred in 91% of patients with leg blood flow > or = 40 ml/min, in 18% with leg flow 30-39 ml/min and in 11% with leg flow < 30 ml/min. Complete relief of pain for at least 6 weeks occurred in 92% of patients in whose limbs the blood flow on day 8 was greater than 50 ml/min but only in 6% with blood flow less than 50 ml/min. These results indicate that iloprost increases leg blood flow and that patients likely to respond can be identified from the baseline air plethysmographic measurement of leg blood flow.