Sixty-nine infants at risk of developing retinopathy of prematurity (ROP) were entered into a prospective study to assess the incidence and natural history of the disease. Seventeen infants developed ROP, and in six eyes the disease progressed to stage IV ROP. The natural regression of a pupillary membrane and physiological vitreous haze was not influenced by the onset of ROP. Progression from stage I to stage III was rapid and the rate was influenced by the zone affected. Congestion and tortuosity of vessels in the posterior pole always signified stage III ROP. Progression from stage III to stage IV ROP was slower; it was characterised by the development of vitreoretinopathy, signified by the sudden onset of a vitreous haze. Iris congestion associated with poor mydriasis may be a grave sign indicating imminent retinal detachment. Cicatricial ROP can be divided into retinal and vitreoretinal cicatricial disease directly related to the stage of active disease reached. ROP is characterised by its self limiting nature, but the stage at which it becomes inactive varies and will influence the final outcome.