Objective: To describe an unusual case of development of diabetes mellitus (DM) several years after manifestation of diabetic nephropathy and to review the related literature. Methods:We present a case report, including detailed laboratory and pathologic findings in a 51-year-old man who was diagnosed as having DM several years after presenting with diabetic nephropathy. The pertinent literature is also reviewed. Results: A 51-year-old African American man presented with proteinuria of 4 g/24 h. Past medical history was significant for impaired glucose tolerance diagnosed 2 years previously. Subsequent follow-up demonstrated fasting blood glucose levels ranging from 108 to 123 mg/dL and glycated hemoglobin levels ranging from 5.3 to 5.8%. The patient also had chronic hepatitis C, hypertension, a history of intravenous drug abuse, and a family history of DM and hypertension. On examination of the patient, his blood pressure was 180/90 mm Hg. Funduscopy revealed mild diabetic retinopathy. Work-up was negative for glomerulonephritis, connective tissue disease, vasculitis, or multiple myeloma. Kidney biopsy revealed thickened glomerular basement membranes and diffuse glomerulosclerosis, consistent with diabetic nephropathy. During follow-up, 9 years after presenting with proteinuria and 4 years after diagnosis of biopsy-proven diabetic nephropathy, the patient had a blood glucose level of 890 mg/dL and diabetic ketoacidosis. Conclusion: This case provides one explanation for the natural course of patients who present with "diabetic complications" but have no diabetes. Some of those patients may have "prediabetes" and may manifest with DM during follow-up. We also conclude that hyperglycemia is not the only important factor in the pathogenesis of diabetic nephropathy.