The differential of neonatal mortality rates between infant transports to tertiary and to intermediate neonatal intensive care units (NICUs) was examined based on 8,391 one-time infant transports from community hospitals to tertiary or intermediate NICUs in Southern California in the three-year period 1981-1983. Among the demographic, birth and delivery, and diagnostic characteristics studied, nine were identified to be related significantly to the higher neonatal mortality rate among transports to tertiary NICUs: birthweight, gestational age, necessity of intubation, multiple clinical conditions, presence of cardiac, neurologic, and genitourinary problems, anomalies, and syndromes. Adjusting for differences in the number of cases with necessity of intubation and the presence of the five clinical problems reduced the neonatal mortality ratio of tertiary to intermediate NICUs from 1:56 to 1:01, while adjustment for birthweight and gestational age differences reduced the ratio from 1.56 to 1.54. This analysis indicates that the difference of neonatal mortality between the two levels of NICUs can be explained to a larger extent by the higher proportion of infants requiring intubation with serious clinical problems. Birthweight and gestational age played only a minor role in this respect.