In an open prospective study the efficacy and tolerance of imipenem/cilastatin was investigated in 24 critically ill patients on mechanical ventilation with nosocomial respiratory tract infection. Nine patients had previously received antibiotic therapy, eight of them with various other beta-lactam antibiotics which had failed. Imipenem was given in a dose of 1-3 g/24 h over 5-37 (mean 11) days. Seven patients were additionally treated with aminoglycosides, one patient with erythromycin. Pseudomonas aeruginosa (n = 14), Staphylococcus aureus (n = 4), Haemophilus influenzae (n = 4) and Escherichia coli (n = 3) were the potential pathogens most frequently isolated from tracheo-bronchial secretions. All of the isolates were susceptible to imipenem. 91% of the infections without and 77% with involvement of P. aeruginosa were successfully treated. Two patients who had not responded to previous treatment succumbed to the consequences of progressive respiratory distress syndrome. All of the gram-positive and 85% of the gram-negative pathogens (Pseudomonas not included) were eliminated in the course of therapy. By contrast, 64% of the isolates of P. aeruginosa persisted; half of these became imipenem-resistant. Nine patients showed adverse reactions including one case of pseudomembranous colitis or laboratory abnormalities which were all reversible. Imipenem/cilastatin proved highly effective and was relatively well tolerated; it is suitable as a single agent for the initial treatment of nosocomial respiratory tract infections in ventilated patients, although only with limitations in cases of infection due to P. aeruginosa.