Nosocomial infections with coagulase negative staphylococcus (CoNS) are a frequent and significant cause of morbidity in the preterm infant. Infections diagnosed after the first 72 hours of life are arbitrarily deemed to be "nosocomial." There are many difficulties encountered in efforts to evaluate and compare nosocomial sepsis in the NICU. An issue of primary concern is the lack of uniformity in the definition of sepsis in the NICU. Based on the frequency of positive blood cultures in infants less than 1000 g, it appears reasonable to evaluate methods for the prevention of nosocomial sepsis. These include prophylactic antibiotic administration, antiseptic impregnated catheters, and the use of an antibiotic lock technique.