We have previously conducted a small treatment study on staphylococcus toxoid in fibromyalgia (FM) and chronic fatigue syndrome (CFS). The aim of the present study was to further assess the efficacy of the staphylococcus toxoid preparation Staphypan Berna (SB) during 6 months in FM/CFS patients. One hundred consecutively referred patients fulfilling the ACR criteria for FM and the 1994 CDC criteria for CFS were randomised to receive active drug or placebo. Treatment included weekly injections containing 0.1 ml, 0.2 ml, 0.3 ml, 0.4 ml, 0.6 ml, 0.8 ml, 0.9 ml, and 1.0 ml SB or coloured sterile water, followed by booster doses given 4-weekly until endpoint. Main outcome measures were the proportion of responders according to global ratings and the proportion of patients with a symptom reduction of > or =50% on a 15-item subscale derived from the comprehensive psychopathological rating scale (CPRS). The treatment was well tolerated. Intention-to-treat analysis showed 32/49 (65%) responders in the SB group compared to 9/49 (18%) in the placebo group (P<0.001). Sixteen patients (33%) in the SB group reduced their CPRS scores by at least 50% compared to five patients (10%) in the placebo group (P< 0.01). Mean change score on the CPRS (95% confidence interval) was 10.0 (6.7-13.3) in the SB group and 3.9 (1.1-6.6) in the placebo group (P<0.01). An increase in CPRS symptoms at withdrawal was noted in the SB group. In conclusion, treatment with staphylococcus toxoid injections over 6 months led to significant improvement in patients with FM and CFS. Maintenance treatment is required to prevent relapse.