The incidence and severity of retinopathy of prematurity (ROP) were studied prospectively in a geographically defined area of the East Midlands. Over 23 months, 505 infants weighing 1700 g or less at birth to mothers resident in this area were studied. Acute ROP developed in 248 (49.1%) and there was a significant association between short gestation and low birthweight and greater severity. Most acute ROP underwent complete resolution; only stage 3/4 disease did not and cicatricial sequelae subsequently developed in 5 infants, 23.8% of this group. No infant was blind because of ROP. Black infants had shorter gestations yet were less likely to have any stage of ROP. Although Asians were not smaller than their caucasian counterparts, severe (stage 3/4) ROP was more likely (14.1% vs 2.7% for caucasians); this difference was largely due to their better survival. Since 505 of the available 547 infants in this geographically defined area were examined, it was possible to calculate the incidence of acute and cicatricial ROP on a community population basis. There are dangers in attempting to deduce the incidence of acute ROP from infrequent eye examinations, and in comparing community and hospital-based reports.