24 hour infusion of iloprost was compared with placebo infusion in 19 patients with stable intermittent claudication using a double blind, balanced crossover design. Despite significant inhibition of platelet aggregation to ADP and collagen (p less than 0.001) and the typical cardiovascular and gastrointestinal side effects, there was no significant effect on treadmill exercise times at any time up to 6 weeks after infusion. The 95% confidence limits indicated that an improvement of more than 25% was unlikely to occur. No significant changes in B thromboglobulin, platelet aggregate ratio, bleeding time, whole blood viscosity and euglobulin clot lysis time were demonstrated.