Two hundred thirty subjects with insulin-dependent diabetes were followed up longitudinally by measuring glycohemoglobin values to relate glucose control with renal and retinal complications. Subjects with long-term poor control (glycohemoglobin values greater than 1.5 times the upper limit of normal) had 3.6 times the prevalence of microalbuminuria and 2.5 times the prevalence of level 3 to 6 retinopathy than that found in subjects with long-term good control (glycohemoglobin values within 1.33 times the upper limit of normal). Variables related to kidney damage were glucose control and, to a lesser degree, duration of diabetes. Variables related to eye disease were, in descending order of significance, duration of diabetes, glucose control, and age. No subject whose mean glycohemoglobin value was consistently within 1.1 times the upper limit of normal had retinopathy or microalbuminuria. In contrast, when the mean glycohemoglobin value was more than 1.5 times the upper limit of normal, 24 (29%) of 82 subjects had microalbuminuria and 30 (37%) of 82 had level 3 to 6 retinopathy.