Abstract Therapy trials with bacterial compounds in irritable bowel syndrome (IBS) have produced conflicting results. This study was performed in 1988 and 1989, and was re-analysed according to current IBS standards. Two hundred ninety-seven patients with lower abdominal symptoms diagnosed as IBS were treated for 8 weeks by the compound ProSymbioflor® (Symbiopharm GmbH, Herborn, Germany), an autolysate of cells and cell fragments of Enterococcus faecalis and Escherichia coli, or placebo in a double-blinded, randomized fashion. Patients were seen weekly by the physician, who assessed the presence of core IBS symptoms. Responders had at least a 50% decrease in global symptom score (GSS) and in abdominal pain score (APS) reports at ≥1 visit during treatment. The responder rate in GSS to the drug was 102/149 (68.5%) in comparison to placebo with 56/148 (37.8%) (P < 0.001), the improvement in APS was 108/149 (72.5%) and 66/148 (44.6%) respectively (P = 0.001). The number-needed-to-treat was 3.27 for GSS and 3.59 for the APS report. Kaplan–Meier analysis revealed a mean response time of 4–5 weeks for active treatment and more than 8 weeks for placebo (P < 0.0001). Treatment of IBS with the bacterial lysate ProSymbioflor is effective and superior to placebo in reducing typical symptoms of IBS patients seen by general practitioners.