Eighteen institutions collaborated in evaluating the comparative efficacy of combined ampicillin and gentamicin therapy with and without intrathecal administration of gentamicin on the clinical and bacteriologic responses of 117 infants with meningitis caused by gram-negative enteric bacteria. There was a random distribution of patients within the two study groups with regard to age on enrollment, birth weight, sex, race, number of infants greater than 30 days of age, the etiologic agent, and their antimicrobial susceptibilities. There were no statistically significant differences (P greater than 0.05) in mortality, morbidity, or days that cerebrospinal fluid cultures remained positive among the infants in the two treatment groups. The case fatality rate for all patients was 32%; that for full-term infants (18%) was significantly lower (P less than 0.01) than that for low-birth-weight infants (45%) or for the patients greater than 30 days of age (48%). Fifty-one of the 80 (64%) survivors were assessed as normal on follow-up examinations performed up to four years after illness.