The incidence, cause, and outcome of sepsis and the white blood cell response were studied in 6315 infants born in a regional perinatal unit. The incidence of neonatal sepsis was 6.5 per 1000 live births. Congenital sepsis (12 cases) was overwhelming, with associated maternal infection (92%), neutropenia (75%), and high rate of mortality (50%). The most common organism was Escherichia coli (58%). Gestational age and birth weight were similar in survivors and nonsurvivors. There was a strong correlation between total white blood cell count and both mature and immature neutrophil counts in survivors but this correlation decreased substantially in neonates that died. Analysis of variance indicated that the means for polymorphonuclear leukocyte and immature neutrophil counts were significantly higher in survivors. Nosocomial sepsis (38 cases) occurred in premature low birth weight infants receiving invasive, intensive care. The most common organism was Staphylococcus epidermidis (76%). Total white blood cell, polymorphonuclear leukocyte, and immature neutrophil counts rose significantly in response to sepsis. None died. Prevention of congenital sepsis requires methods to detect early maternal-fetal infection. Providing granulocytes to neutropenic neonates with congenital sepsis might improve outcome.