In the period 1985-1991, 21,675 infants were born at the University Hospital of Copenhagen, Hvidovre Hospital, Denmark. Two hundred and twenty-four infants (10.3%) with birth weights < or = 1500 g and gestational ages < or = 32 completed weeks were transferred to the neonatal intensive care unit of the hospital. One hundred and eighty survived to at least 8 weeks of age and 170 had eye examinations. Forty-five of the 170 infants examined (26.5%) had retinopathy of prematurity (ROP) and 18 (40%) of these developed blindness or severely impaired vision, a higher incidence than reported in other studies. Significant differences were found between infants with and without ROP for: birth weight, gestational age, Apgar score at 1 min, resuscitation, ventilator treatment, duration of supplementary oxygen, severe complications in the neonatal period and sequels from the central nervous system. Statistical analysis, corrected for correlations, showed that the occurrence of ROP was related significantly to early intubation, hypotension, persistent ductus arteriosus and necrotizing enterocolitis.