A randomised prospective clinico-laboratory evaluation of the efficacy of ornidazole versus clindamycin in anaerobic infections was performed in 140 patients; 67 were given ornidazole and 73 received clindamycin. Patients were mainly suffering from peritonitis, pelvic cellulitis, endometritis, soft tissue infections and abdominal abscesses, which were distributed rather equally in both groups. Ornidazole was administered at a dose of 500 mg every 12 h i.v. or/and orally, and clindamycin 600 mg every 8 h i.v. for 7–60 days. In pus cultures, Escherichia coli and Bacteroides fragilis were the main isolates. The coexistence of aerobes necessitated the addition of an aminoglycoside in 111 patients, while six times chloramphenicol had proved ineffective against anaerobes. Between the two groups no statistically significant difference was found in the excellent response rate, although the overall cure rate was superior in the ornidazole group (80.6 vs. 68.5%), with a prompt response within less than 48 h in the case of ornidazole. Side effects necessitating discontinuation of chemotherapy included severe nausea in 1 patient treated with ornidazole and diarrhea in 8 patients given clindamyci