In a double-blind, placebo cross-over study in 14 patients with Raynaud's disease, oral ketanserin 40 mg b.d. for two months reduced the number of digital ischaemic attacks in 36% of the patients, with no difference between primary or secondary cases. Intraplatelet and circulating serotonin concentrations were significantly higher in patients than in controls. During treatment there was a significant decrease in intraplatelet serotonin, but no change in circulating serotonin or in the amount of serotonin released by platelets in vitro. The results suggest that oral ketanserin is of minor help in Raynaud's phenomenon and they support a role for serotonin in the disorder.