A total of 497 patients aged 18 to 56 years with insulin-dependent diabetes mellitus for 1 to 15 years were randomly assigned to take oral sorbinil or placebo and followed up for a median of 41 months. The percentage of patients whose retinopathy severity grade at maximum follow-up had worsened by two or more levels was not significantly different between the two treatment groups (28% in the sorbinil group and 32% in the placebo group, P = .344). The number of microaneurysms increased at a slightly slower rate in the sorbinil group than in the placebo group, with statistically significant differences at 21 (P = .046) and 30 (P = .039) months but not at the maximum follow-up (P = .156). About 7% of the patients assigned sorbinil developed a hypersensitivity reaction in the first 3 months. On the basis of these results, it is unlikely that sorbinil administered at a dosage of 250 mg daily for 3 years has a clinically important effect on the course of retinopathy in adults with insulin-dependent diabetes of moderate duration. Our data are consistent, however, with a slightly slower progression rate in the microaneurysm count among patients assigned to take sorbinil, a finding of uncertain clinical importance.