A one year prospective surveillance of nosocomial infections (NI) in a neonatal intensive care unit (NICU) was performed. Among 229 neonates the infection rate was 27.1%, the infection proportion 20.1%, and the incidence density 21.9 infections per 1000 patient days. Infants were stratified into four birth weight categories. Degrees of infection ranged from 44.4% in the < or = 1000 g group to 10.1% in the > 2500 g group. Differences between the groups were statistically significant (P < 0.01). The mean birth weight of infants with NI was significantly lower than that of infants without NI (1711 g, SD +/- 841 g vs. 2213 g, SD +/- 896 g; P < 0.01). Mortality of < or = 1000 g babies was 44.4 and 7.6% in > 2500 g neonates. Major sites of infection were pneumonia (32.3%), blood-stream infections (27.4%), infections of the skin, and surgical site infections (11.3% each). The predominant pathogen was Staphylococcus aureus (24.2%) whilst Gram-negative bacteria accounted for 22.7% of the total. Other major infective agents were Staphylococcus epidermidis, Escherichia coli, and Group B streptococci. It is concluded, that low birth weight was a major risk factor for the acquisition of NI in the observed NICU population.