Men and women eligible for the SRT had diabetes for 1 to 15 years and were between ages 18 and 56 at the time of enrollment. They had begun taking insulin before their 41st birthday. Their hemoglobin A1c value was within the diabetic range. On retinal examination, they showed no evidence of or only very mild retinopathy, with no more than five microaneurysms per eye. Women were postmenopausal, sterile, or had an IUD in place.