The occurrence of congenital and nosocomial bacterial septicaemia has been documented by identifying the number of positive blood cultures by reviewing the laboratory and clinical records of 394 very low birth weight infants who were consecutively admitted to a neonatal intensive care unit over a 40-month period. The incidence of congenital septicaemia was 6% and of nosocomial septicaemia 17%. The commonest causes of congenital infection were Streptococcus agalactiae Staphylococcus epidermidis and Enterococcus faecalis (each in 18% of cases). The commonest cause of nosocomial infection was S. epidermidis (51% of cases), except in infants of birth weight less than 750 g. Risk factors for nosocomial infection were extremely low birth weight, very preterm birth and prolonged ventilation. Nosocomial infection was associated with significantly lengthened hospital admission.