The results of several published studies suggest that bacterial infection plays an important role in the mortality of Neonatal Intensive Care Unit (NICU) patients. To evaluate more thoroughly this role we have routinely performed postmortem cultures of blood and cerebrospinal fluid on all infants who die in our NICU. During a 5-year review period 25.9% of infants who died had one or both cultures positive for an organism considered to be pathogenic in the neonate. The organisms most commonly cultured were Klebsiella pneumoniae, Escherichia coli and Group B Streptococcus. Of those infants who had positive postmortem cultures and who had an autopsy, 95% had histologic evidence of infection. Forty-three percent of our infants with positive postmortem cultures were not suspected of having a serious infection and had not received antibiotics before death. Postmortem cultures appeared to be helpful in identifying NICU patients in whom serious infection may have played a role in mortality. Unsuspected serious infection is a common finding in infants who die in a neonatal intensive care setting.