This population-based, historical, follow-up study analysed possible risk factors for retinopathy of prematurity (ROP) and resulting visual impairment in newborns over a period of 3 1/2 years in the County of Northern Jutland, Denmark. The study subjects were 141 infants with birth weight < or = 1500 grams and gestational age < 35 completed weeks who survived to a first eye examination at 5 weeks. The incidence of ROP (all stages) was 18%, and the frequency of severe visual impairment from ROP was 14/100,000 live births. A logistic regression analysis identified low gestational age, multiple births, continuous oxygen treatment, and male sex as risk factors for ROP. The main factor in the prevention of ROP is to prevent preterm births.