OBJECTIVE: To determine the incidence and evaluate the antimicrobial-susceptibility patterns of bacterial infections in our neonatal units. DESIGN: Retrospective surveillance study. SETTING: The neonatal units of the Hadassah University Hospitals, Jerusalem, Israel. PATIENTS: All newborns admitted from January 1994 through February 1999. METHODS: The records of all patients with positive blood and cerebrospinal fluid cultures were reviewed. Bacteremia was considered early-onset (vertical) when occurring within the first 72 hours of life and late-onset (nosocomial) when occurring later. The prevalence and antibiotic-resistance patterns of vertically transmitted and nosocomially acquired strains were compared and studied over time. RESULTS: 219 of 35,691 newborn infants had at least one episode of bacteremia (6.13/1,000 live births). There were 305 identified organisms, of which 21% (1.29/1,000 live births) were considered vertically transmitted and 79% nosocomially acquired. The most common organism causing early-onset disease (29.2%) was group B streptococcus (0.38/1,000 live births), whereas coagulase-negative staphylococci (51%) were the most prevalent in lateonset disease. All gram-positive bacteria were susceptible to vancomycin. Most gram-positive organisms other than staphylococci were susceptible to ampicillin. Gram-negative organisms represented 31% of all isolates. Generally, there was a trend of increasing resistance to commonly used antibiotics among nosocomially acquired gram-negative organisms, compared to those vertically transmitted, with statistically significant differences for ampicillin and mezlocillin (P<.05 and P<.01, respectively). Over the years, a trend toward an increasing resistance to antibiotics was observed among gram-negative organisms. CONCLUSIONS: The trend of increasing bacterial resistance to commonly used antibiotics necessitates the implementation of a rational empirical treatment strategy, based on local susceptibility data, reserving certain agents for emerging resistant pathogens.