The onset of diabetes relative to clinical diagnosis was estimated in Egyptians with non-insulin-dependent diabetes mellitus (NIDDM) based on the relationship between retinopathy and duration of diabetes. Between July 1992 and October 1993 the Diabetes in Egypt (DIE) Project performed a cross-sectional, population-based survey with clinical and laboratory follow-up to describe the prevalence of microvascular, neuropathic, and macrovascular complications among Egyptians > or = 20 years of age with diagnosed diabetes, previously undiagnosed diabetes, impaired glucose tolerance, and normal glucose tolerance. The sample of persons with diabetes diagnosed prior to the survey had medical examinations which included a dilated eye examination and retinal photographs. Generalized linear models were used to relate the probability of retinopathy to duration of diabetes. Among 218 persons with diabetes diagnosed prior to the DIE project, 87 (40%) had diabetic retinopathy. The onset of retinopathy was estimated to occur 2.6 years (p = 0.04) prior to clinical diagnosis. The estimated annual incidence of retinopathy was 5% and the estimated prevalence at the time of clinical diagnosis of diabetes was 12%. On the basis of reports that retinopathy does not occur until approximately 5 years after the development of diabetes, the onset of NIDDM was estimated to occur 7.6 years prior to its clinical diagnosis. This estimate of the onset of NIDDM in Egyptians is comparable to other estimates reported for US and Australian populations.