The worldwide experience with imipenem/cilastatin as of November 7, 1983, in the intravenous therapy for severe and moderately severe infections of the lower respiratory tract is reviewed. Of 204 assessable patients treated in 77 studies conducted by 70 investigative groups (43 in the United States, 27 in other countries), 173 (85%) were cured of their infections or showed improvement. Imipenem was tested against 289 of 303 bacterial pathogens isolated before therapy, and 284 (98%) were found to be susceptible. Principal pathogens were Pseudomonas aeruginosa, Streptococcus pneumoniae, Klebsiella species, Haemophilus influenzae, Escherichia coli, and Staphylococcus aureus. A total of 76% of infecting pathogens were eradicated during therapy. Of 54 imipenem-susceptible infecting strains of P. aeruginosa, however, 57% were eradicated, 19% acquired resistance to imipenem, and 17% were replaced by new resistant strains of P. aeruginosa. Higher doses of imipenem/cilastatin and/or combined therapy with an aminoglycoside may improve these results. Imipenem/cilastatin compares well with the most active agents available for therapy for lower respiratory tract infections.