The aim of the study was to evaluate in a double-blind manner the effect of the long-acting 5-hydroxytryptamine 2 (5-HT2)-receptor blocker Ritanserin on clinical symptoms in patients with fibromyalgia syndrome (FM) and on production of antibodies to serotonin, gangliosides and phospholipids, recently shown to have a high incidence in this disease. Fifty-one female patients with typical FM were included in the 16-week study: 24 received Ritanserin and 27 received a placebo. Antibodies to 5-HT, gangliosides (Gm1) and phospholipids (thromboplastin) were determined by enzyme-linked immunosorbent assay at day 0 and at the end of week 16. The psychological and physical status, including tender points, of the patients was evaluated at day 0 and at the end of weeks 4 and 16. At the end of the study, there was an improvement (p < 0.05) in feeling refreshed in the morning in the Ritanserin-treated group and headache was also significantly improved compared with the placebo group. There was no difference in pain, fatigue, sleep, morning stiffness, anxiety and tender point counts in the Ritanserin and placebo groups. Fifty-one per cent of the 51 patients had at least one of the three antibodies to 5-HT, Gm1 and phospholipids. The incidence and activity of these antibodies were not influenced by Ritanserin or placebo. The observation that Ritanserin has only a small effect on clinical symptoms indicates that disturbances in serotonin metabolism or uptake may be only one factor in the pathogenesis of the disease. The high incidence of a defined autoantibody pattern in FM could again be confirmed in this study. However, it remains speculative whether immunological reactions are, indeed, involved.