OBJECTIVE: To ascertain whether insulin-like growth factor 1 (IGF1) is associated with retinopathy of prematurity (ROP) and is a useful predictor of the disease. Although its aetiopathogenesis is multifactorial, development of the disease appears to be related to a deficiency in IGF1, a hormone that acts together with vascular endothelial growth factor in the normal angiogenesis in the retina. DESIGN: Prospective study for a 30-month period. PARTICIPANTS: A total of 74 premature newborn babies, of less than 1500 g and/or 32 weeks' gestational age or less. TESTING: To determine the development and severity of ROP. MAIN OUTCOME MEASURES: Serum levels of IGF1 were measured once a week from birth until 40 weeks corrected gestational age in each subject. RESULTS: Of our subjects, 32.4% developed some form of ROP, and all those ROP patients had the following characteristics at birth (median +/- standard deviation scores): low weight (1098 +/- 188 vs. 1393 +/- 285 g), short length (36.74 +/- 1.77 vs. 38.89 +/- 3.08 cm), small cranial perimeter (26.03 +/- 1.74 vs. 27.93 +/- 1.81 cm) and young gestational age (29.7 +/- 1.78 vs. 31.3 +/- 1.79 weeks) (p < 0.05). Other factors previously associated with ROP that were also observed with statistically significant frequency in our ROP patients were bronchopulmonary dysplasia, intracranial haemorrhage, the need for erythrocyte transfusion or treatment with erythropoietin and sepsis (all p < 0.05). Levels of IGF1 at the 3rd week post-partum, independent of gestational age at birth, were clearly lower in the group who developed ROP (29.13 vs. 43.16 ng/mL, p < 0.05). A value of 30 ng/mL of IGF1 in the third week post-partum was found to have a 90% sensitivity in the diagnosis of ROP. A rapid rise in IGF1 levels between the 3rd and 5th weeks appeared to be related to the development of a higher stage of ROP. CONCLUSION: Determination of IGF1 serum levels in the 3rd week post-partum, independent of gestational age at birth, provides a sufficient and reliable prognostic tool and allows the identification of a group of patients at high risk of developing the disease.