Growth factors such as growth hormone and insulin-like growth factor 1 (IGF-1) may be important in the pathogenesis of diabetic retinopathy. We measured serum IGF-1 in 371 diabetic patients attending a diabetic retinopathy clinic and in 73 non-diabetic control subjects. No significant difference was observed in IGF-1 level between the diabetic and control groups (168 +/- 3.9 vs 177 +/- 7.4 micrograms/l [mean +/- SE]). Within the diabetic group, there was no difference between patients with no retinopathy and those with proliferative change (198.7 +/- 8.8 vs 190.5 +/- 11 micrograms/l). After adjusting for differences in age, duration of diabetes, and presence of proteinuria, only the inactive previously proliferative group showed any significant difference from the other patient subgroups (151.8 +/- 11.5 micrograms/l; p less than 0.05). Serum IGF-1 correlated with age in the control group (r = 0.49; p less than 0.001) and to a lesser extent in the diabetic group (r = -0.23; p less than 0.05). IGF-1 levels were higher in patients with proteinuria than in those without proteinuria (196.8 +/- 10.3 vs 138.8 +/- 4.4 micrograms/l; p less than 0.001).