A chemiluminescence (CL) microassay was used to evaluate polymorphonuclear leukocyte (PMN) function in premature newborn infants longitudinally during a 2-month period and in healthy adult control subjects. At postnatal ages of 12, 26, 40 and 54 days the infants' mean peak CL activity was significantly lower than that of the adults. Infants with one or more low CL responses were more severely ill than those with normal CL activity. The infants with low CL responses had longer hospital stays and a higher frequency of serious infections, as well as more days of level 3 care, antimicrobial therapy, supplemental oxygen, assisted ventilation, and total parenteral nutrition. The PMN CL activity before, during, and after episodes of serious infection did not differ. In addition, a high frequency of depressed CL activity was observed at the time of infection. Our findings are consistent with previous studies suggesting that defective PMN oxidative metabolic responses are more common in neonates undergoing stress. Our results further suggest that defective PMN function may persist for the first 2 months of life and during the course of serious infection. Enhancement of PMN host defense may be an important strategy in the management of neonatal sepsis.