Purpose: This study aimed to investigate the relationship between rate of postnatal weight gain and severity of retinopathy of prematurity (ROP). Methods: All infants (n = 111) screened for ROP at a single tertiary intensive care nursery over a 2-year period with an estimated gestational age of 30 weeks or less and follow-up to at least 42 weeks' postconception were included. The authors performed a retrospective review of records and statistical analysis of risk factors for ROP using multivariate analysis. Results: Infants with severe (stage 3 or greater) ROP gained an average 10.9 g/kg per day in the first 6 weeks of life, compared to a mean of 9.6 g/kg per day for those with mild or no ROP (P = .04). By multiple regression, which included birth weight, gestational age, and 9 other reported risk factors, there was an association between rate of postnatal weight gain and severity of ROP (P = .02). By stepwise regression, 4 variables were associated with ROP severity: estimated gestational age at birth (P = .002), rate of postnatal weight gain (P = .0002), volume of transfused erythrocytes (P = .0001), and culture-proven sepsis (P = .02). Conclusion: Poor postnatal weight gain is a risk factor for the development of severe (stage 3 or greater) ROP. Ophthalmologists should take note of those infants who gain less than 50% of their birth weight in the first 6 weeks of life. (J AAPOS 2000;4:343-7).