During a 12-month study of the causes of and risk factors for nosocomial sepsis in a neonatal intensive care unit (NICU), we detected 23 episodes of nosocomial sepsis in 20 of the 155 infants at risk who were hospitalized in the NICU for at least one week. The associated mortality was 20%. Gram-positive organisms accounted for 15 (65%) of the episodes. Low birth weight, multiple gestation, and prolonged hospitalization were significant risk factors for nosocomial sepsis by univariate analysis; together, these three factors correctly predicted 80% of the infants with sepsis and 82% of the control subjects. By logistic regression analysis, however, length of stay was not a significant risk factor, but rather a confounding variable that was highly associated with birth weight. Analysis of risk factors for nosocomial sepsis showed that previous antibiotic therapy placed an infant at risk for candidemia; assisted ventilation was a risk factor for sepsis caused by group D Streptococcus and Candida albicans. Sepsis was related to infected or malfunctioning intravascular catheters in nine of the 20 infants with sepsis. Further investigation to determine strategies for preventing nosocomial septicemia in the low birth weight infant is warranted.