216 patients with insulin-dependent diabetes mellitus were studied by retinal photography, and the absence or presence of retinopathy was related to the mean of serial glycosylated haemoglobin measurements (mean HbA1) carried out every 3 months during the previous 6 years. 122 patients had no diabetic retinopathy, 86 had background retinopathy, and 8 proliferative retinopathy. Mean HbA1 levels showed a strong correlation with increasingly severe grades of retinopathy, even when differences in duration of diabetes were taken into account. Proliferative retinopathy was seen only in patients with mean HbA1 above 10%. These results support the view that the development of diabetic retinopathy is related to long-term glycaemic control and emphasise the desirability, and possible benefit, of achieving control as close to normal as is possible for each individual patient.