To assess the degree of perinatal regionalization, maternal and infant records were reviewed for all very low birth weight (501-1500 g) infants born in calendar years 1985-1986 to residents of the primarily rural North Central Perinatal Region of Illinois. Seventy-one percent of mothers who were expected to deliver in non-center hospitals could have realistically been referred to perinatal centers for delivery. Ninety-four percent of realistic antenatal referrals actually occurred. In Peoria, 79% of mothers expected to deliver very low birth weight infants at non-center hospitals could realistically have been referred, to the center for delivery, and all such referrals actually occurred. Because a goal of regionalization is to deliver certain high-risk women in centers, the fact that 94% of rural and 100% of urban realistic antenatal referrals actually occurred suggests that the North Central Perinatal Region is well regionalized. Other centers should study the site of delivery of this or other well-defined regional cohorts of high-risk pregnancies to quantitate how well perinatal care is regionalized in their respective areas.