The effect of a change in the first line antibiotic treatment in a neonatal unit was studied. A total of 238 neonates (G1), admitted between 1 January and 31 July 1989, and treated with gentamicin, were compared with 398 (G2) admitted between 1 August 1989 and 31 July 1990 who received amikacin, in the combination of ampicillin plus an aminoglycoside. This change was implemented in an attempt to prevent the spread of an epidemic strain of Enterobacter cloacae resistant to third generation cephalosporins and all aminoglycosides, except amikacin. The change in treatment had no effect on the incidence of nosocomial infections [19.7% (G1) vs. 16.3% (G2) RR = 1.21 (0.86-1.70)], but the proportion of patients with nosocomial infections caused by the E. cloacae decreased (6.3% vs. 2.0% RR 3.14 CI 1.35-7.28). Certain trends in the bacterial ecology emerged: E. aerogenes and Enterococci increased in G2. The proportion of gentamicin-resistant strains such as E. cloacae or Staphylococci decreased and there was no increase in aminoglycoside-resistant strains, except in Escherichia coli, in which resistance to amikacin rose from 0 to 3%. This study illustrates the influence of antimicrobial therapy on the species and the resistance of strains isolated in nosocomial infections. It also highlights the need for epidemiologic surveillance, and poses the question of how best to modify antibiotic policy.