In a six-month prospective study involving 60 noninsulin-dependent diabetics, we evaluated the effect of change to conventional insulin therapy on the pre-existing retinopathy. Ten of 15 patients in group 1 (background or preproliferative diabetic retinopathy treated with insulin) compared to one of 15 patients in group 2 (background or preproliferative diabetic retinopathy treated with hypoglycemic agents) had progression of retinopathy. Worsening of retinopathy was related significantly only to the age of the patients (r = .752, P < .001). None of the 15 patients each in groups 3 and 4 (no retinopathy treated with insulin injections [group 3] or oral hypoglycemic agents [group 4]) developed retinopathy.