In a randomized, double-blind study, the selective and specific S2-serotonergic receptor antagonist, ketanserin was compared with placebo in 24 patients with systemic sclerosis. Following a 6-week placebo washout period, patients were randomly allocated to receive ketanserin or placebo for 6 months. Ketanserin failed to produce a greater improvement than placebo in functional and objective clinical signs and symptoms as well as in most subjective assessments. However, in a global rating by the physician ketanserin was superior to placebo. No difference in the frequency or severity of side-effects was found. The results cast doubt on the hypothesis that serotonin may be a major contributing factor in the pathophysiology of systemic sclerosis.