The clinical and bacteriological efficacy and the tolerability of meropenem versus imipenem/cilastatin (both 1 g t.i.d.) in severe nosocomial infections were compared in a multicentre, randomised, nonblinded study. A total of 151 patients were recruited; 133 (66 meropenem, 67 imipenem/cilastatin) were clinically evaluable and 84 (42 meropenem, 42 imipenem/cilastatin) bacteriologically evaluable. Most clinically evaluable patients (90%) were in intensive care units, required mechanical ventilation (72%), and had received previous antibiotic therapy (62%). The mean (+/- SD) APACHE II score was 15.2 (+/- 6.6) in the meropenem group and 17.8 (+/- 6.8) in the imipenem/cilastatin group. The primary infections were nosocomial lower respiratory tract infections (56% of patients), intra-abdominal infections (15%), septicaemia (21%), skin/skin structure infections (5%), and complicated urinary tract infections (3%); 35% of the patients had two or more infections. There was no significant difference between the meropenem and imipenem/cilastatin groups in the rates of satisfactory clinical (weighted percentage 87% vs. 74%) or bacteriological (weighted percentage 79% vs. 71%) response. There was a slightly higher rate of clinical success with meropenem against primary or secondary lower respiratory tract infection (89% vs. 76%). Drug-related adverse events occurred in 17% and 15% of meropenem and imipenem/cilastatin patients, respectively. Meropenem (1 g t.i.d.) was as efficacious as the same dose of imipenem/cilastatin in this setting, and both drugs were well tolerated.