Records have been kept prospectively in our institution since 1928 of all positive blood cultures taken from neonates. Using a modification of objective Centers for Disease Control criteria to define sepsis, we reviewed the records of all neonates with positive blood cultures for the years 1979 to 1988 inclusive and found 270 cases of sepsis. The sepsis rate for infants less than or equal to 30 days of age was 2.7 cases/1000 live births, with a mortality rate from sepsis of 15.9%. There was an increase in sepsis due to commensal species (CS) over the period (P less than 0.007). The number of infants in the nursery who developed sepsis when more than 30 days of age also increased (P less than 0.002), as did the rate of sepsis from CS in this group (P less than 0.001). Isolation of CS from the blood with fulfillment of the modified Centers for Disease Control criteria was associated with a 13.7% mortality rate, whereas isolation of CS without fulfillment was associated with a 4% rate (P less than 0.01).