In a randomised and coordinated multicentre study, 143 patients with severe infections received treatment with either imipenem/cilastatin (72; I/C group) or cefotaxime combined with metronidazole and optional cloxacillin (71; CX/M/CL group). 67 patients in the I/C group and 65 in the CX/M/CL group were evaluable for clinical efficacy. 35 (I/C) and 44 (CX/M/CL) patients had bacteriologically documented infections. The clinical cure rate was 91% for I/C and 94% for CX/M/CL, and the bacteriological efficacy (eradicated strains) was 86% and 81%, respectively. Local reactions (phlebitis) were frequent in each group (I/C 18% and CX/M/CL 25%). Clinical side effects other than phlebitis (I/C 17% vs. CX/M/CL 13%) were mostly mild. Diarrhoea did not occur in I/C group. Laboratory untoward effects were mild and infrequent in each group. Reinfections of the urinary tract and superinfections at all sites prevailed in the CX/M/CL group (9 vs. 6 and 6 vs. 2, respectively). The superinfections were mostly mild. The efficacy and safety of I/C in severe infections were comparable to those of CX/M/CL.