Hospital costs for 61 neonates with acquired nosocomial infection were compared with 61 matched, uninfected controls. The increase in length of hospital stay (+23 per cent) added to the number of laboratory tests increase total hospitalization costs to 32 per cent. An additional US $1250 for each case of infection is the cost to the Social Security system. The importance of this increase, contrasted with the generally benign nature of the infections studied, justifies the maintenance of high standards of quality in neonatal care.