Data from 196 infants were analyzed who had been treated for neonatal septicemia and/or meningitis between 1962-1974 (n = 88) and 1975-1985 (n = 108). In addition to an increase in the incidence of septicemia (1962-1974: 0.88 cases/1000 live births/year; 1975-1985: 1.8 cases/1000 live births/year) there was also a change in the pattern of infection. Group B streptococcal infections were first observed in 1975. Infections with Escherichia coli increased (1962-1974: 0.25 cases/1000 live births/year; 1975-1985: 0.65 cases/1000 live births/year). Although the incidence of meningitis was similar in both periods (1962-1985: 0.45 cases/1000 live births/year) the relative number of cases declined (1962-1974: 51 of 88 patients; 1975-1985 25 of 108). Mortality also decreased during the second period (1962-1974: 53%; 1975-1985: 29%). All infants were primarily treated with a combination of ampicillin and gentamicin. The decision to discontinue this therapy was based on the clinical course of the patient and the results of culture and susceptibility studies. Ampicillin and/or gentamicin were effective in vitro against all microorganisms which caused septicemia and/or meningitis within the first four days of life. In contrast this antimicrobial combination was only active in vitro against 77% of the pathogens isolated after this time period.