Twenty-two patients admitted to the ICU with a severe nosocomial infection caused by multi-resistant Gram-negative bacilli were treated with imipenem combined with cilastatin. We treated nine cases of meningo-ventriculitis, eight cases of septicaemia, four cases of mediastinitis, and one case of pneumonia. The bacteria responsible were Acinetobacter spp. (10), Pseudomonas aeruginosa (5), Enterobacter cloacae (5), Klebsiella pneumoniae (3), Proteus spp. (2), Streptococcus spp. (2), Serratia marcescens (1). More than one pathogen was isolated in five cases. The dosages ranged between 1.5 g to 4 g per day by intravenous infusion; the highest doses were used for the treatment of meningitis. The mean duration of treatment was 17 days. An aminoglycoside was combined with imipenem in 18 cases. Cure was obtained in 17 out of the 22 cases. Very rapid sterilization of the CSF in the cases of meningitis and ventriculitis was noted. Two patients died rapidly despite eradication of the bacteria. One case of meningitis relapsed but cure was subsequently obtained with continuation of the same treatment. In three cases of Ps. aeruginosa infection, resistant mutants were isolated from the sites of infection and were responsible for two failures and one colonization. Imipenem appears to be an antibiotic of choice in severe nosocomial infections including meningo-ventriculitis, especially those caused by Acinetobacter spp. and Ps. aeruginosa. It is also one of the few antibiotics active against both streptococci and multi-resistant Gram-negative bacilli. Careful bacteriological monitoring is recommended during treatment.