In a prospective study of transfused neonates, 32 of 262 infants were viruric at greater than 20 days of life. Of 212 neonates whose early status was known, postnatally acquired infection was proven in 21, two of whom were seronegative at birth and were thought to have transfusion-acquired cytomegalovirus (CMV). Maternal transmission of CMV is important in this population as there was 91% seropositivity for CMV at birth among the 21 babies who acquired CMV compared with 55% positivity among 150 babies who did not shed CMV (P less than 0.01). Significantly increased morbidity (increased length of stay in hospital, increased use of antibiotics, and longer duration of antibiotic administration) was found in babies with acquired CMV compared with matched controls who did not become viruric. Significant morbidity and mortality was not restricted to the two seronegative babies with transfusion-acquired CMV. The cost of providing CMV antibody negative blood for this neonatal unit would be less than the cost of providing the extended hospital care needed by the two babies with transfusion-acquired CMV found during this 3 year study.