We evaluated proteinuria to determine the frequency of diabetic nephropathy and to study epidemiological aspects of this disease. We measured 24-hour urinary protein excretion in 152 diabetic patients. We recorded the age and sex of each patient, the duration of diabetes and blood glucose concentration. Proteinuria was diagnosed in 28% of the diabetic patients. The frequency of proteinuria was higher in men (33%) than in women (19%). The highest frequency (37.5%) was observed in subjects aged 70 years or over. In these patients, the higher frequency of proteinuria was associated with a longer duration of diabetes. However, proteinuria was detected in 28% of patients with diabetes diagnosed less than one year previously, suggesting a long period of undiagnosed diabetes in these subjects. Finally, proteinuria was more frequent in patients treated with insulin (42%) than in those treated orally (25%). Thus, diabetic nephropathy is a frequent complication in black diabetic patients in Cotonou. As hemodialysis and kidney transplantation are very expensive and access to these treatments is limited in developing countries, preventive measures based on optimizing patient management and the early diagnosis of diabetes and its complications are required.