Iloprost, a stable prostacyclin analogue, was given by intravenous infusion to 29 patients with severe Raynaud's phenomenon, 26 of whom had systemic sclerosis (SS), and compared with placebo infusion in a double blind crossover trial. Iloprost significantly lessened the number and the severity of attacks compared with placebo. Nine patients expressed a preference for effectiveness of treatment, eight of these in favour of Iloprost. Thermography failed to show any long term effect of Iloprost. Side effects of headache, flushing, nausea, and vomiting were common, and the inconvenience of intravenous administration may limit its routine use.