We have developed a classification system for the acute phases of retinopathy of prematurity based on more than 13,000 ophthalmoscopic examinations of more than 3,400 premature infants between 1968 and 1982. Two forms of the active disease exist. Retinopathy of prematurity is a relatively common mild disease and retinopathy of prematurity plus is characterized by rapid progression and posterior pole vascular tortuosity and dilation. The five grades progress from peripheral vascular abnormalities (Grade 1) through a demarcation line (Grade 2) and extraretinal neovascularization (Grade 3) to partial (Grade 4) or total (Grade 5) retinal detachment. The persistence of abnormal retinal vessels during the first year of life is considered "transitional" retinopathy of prematurity unless unequivocal cicatricial changes with macular distortion develop.