Imipenem is the first of a new class of beta-lactam antimicrobial agents with potent in vitro activity against most bacterial pathogens that cause infections in children. We studied, prospectively, the clinical efficacy and toxicity of imipenem/cilastatin in 40 children with proved or suspected bacterial infection. A dose of 100 mg/kg/day of imipenem was given to children younger than 3 years of age, while children older than 3 years of age received 60 mg/kg/day. Twenty-nine organisms were isolated from 26 patients. Infections treated included cellulitis, osteomyelitis, septic arthritis, lymphadenitis, renal infections, wound infections, and pneumonia. Bacteria isolated included Staphylococcus aureus, Streptococcus pyogenes, Haemophilus influenzae, and Pseudomonas aeruginosa. All patients responded favorably to treatment, with defervescence and improvement of symptoms. All of the infecting bacteria were susceptible to imipenem. Imipenem/cilastatin was well tolerated, with no serious side effects, and appeared to be an effective and safe antimicrobial agent in the treatment of the population studied.