The prevalence of diabetes mellitus among patients treated for end-stage renal failure was studied using a questionnaire mailed to all dialysis units of mainland France in 1989. With a response rate of 80.8%, the study population amounted to 12,903 dialysed patients of whom 884 were declared diabetic (6.9%). In a second phase, the study focused on the diabetic patients treated in the 63 largest units (those with at least four diabetic patients). Seven specially trained physicians completed questionnaires after having interviewed the patients and checked their medical records. All this material was reviewed by the same diabetologist. The conflict of diabetes type declared by both sources of information (the nephrologists and the diabetologist) showed a misclassification rate of 31.2%. Using these new data, the prevalence of type 1 diabetes mellitus was estimated at 1.4% of patients on dialysis therapy in mainland France, and 5.5% for type 2 diabetes mellitus. A north-south declining trend was suggested for type 2 diabetes mellitus. Diabetic nephropathy was the only primary renal diagnosis among 93.9% of type 1 diabetic patients, but only for 36.8% of type 2 diabetic patients. Of the latter, 51.6% had a non-diabetic cause of renal failure. These data show that the proportion of diabetics among patients receiving dialysis, while steadily increasing in France, remains lower than in other countries in Europe and in North America. However, the validity of international comparisons depends on diabetes ascertainment. Heterogeneity in selection of patients and in diabetes type classification by dialysis units may account to a considerable degree for the differences between diabetes mellitus prevalence across countries.