Retinopathy of prematurity (ROP) continues to be a problem for some preterm infants who survive the neonatal period. We review changes which have occurred in the premature infant population and methods of ophthalmological examination since the last survey of ROP in the United Kingdom was conducted between 1951 and 1953. We have analysed data from a retrospective study, and from our current prospective survey of ROP in the East Midlands, to determine the age at onset of acute ROP. The results show that the ophthalmoscopically visible signs of ROP develop over a narrow postmenstrual age range suggesting that acute ROP occurs only after the retina and/or its vasculature have reached a certain stage of development.