The hypothesis that both perinatal events and stage of retinal development are important factors in determining the age at onset of retinopathy of prematurity (ROP) was tested by comparing gestational age at birth with postnatal and postconceptional age when ROP (using ICROP) was first seen. The study population consisted of 207 infants (111 placebo (P) treated, 96 vitamin E (E) treated) who developed ROP among a group of 914 premature infants (460 P, 454 E) enrolled in a randomised clinical trial of the effect of prophylactic use of vitamin E at pharmacological serum levels on incidence and severity of retinopathy. The mean postnatal age at onset of retinopathy was delayed in E treated infants compared with P treated infants by 1.4 weeks (t = 4.004, p < 0.0001). For both P and E treated infants postnatal age at onset of ROP (which reflects the state of retinal development at which birth insults occur) and postconceptional age at onset of ROP which defines state of maturity) were correlated with gestational age at birth. This suggests that both the event of premature birth and the extent of retinal development are important in determining when ROP will first be observed.