Twelve female patients with severe secondary Raynaud's phenomenon were treated in a randomized order with both placebo and Iloprost infusions. Infusions were for 5 hours on 3 consecutive days and Iloprost was administered at variable dosage from 1.0 to 3.0 ng/kg/min. A 6-week follow-up period was used between the two sets of infusions. A significant number of patients reported Iloprost had improved Raynaud's symptomatology compared with placebo and this effect lasted for up to 6 weeks. The number of attacks of Raynaud's as recorded by patients in diary books was similarly reduced after Iloprost. Digital and nail-bed blood flows measured by laser-Doppler methods were increased for up to 6 weeks after Iloprost, but not after placebo infusions. Iloprost may be a useful therapeutic agent in the treatment of severe secondary Raynaud's syndrome.