To find out the mode of presentation of lean patients with Diabetes Mellitus, to Study the clinical profile of lean diabetics with a special reference to stimulated C-Peptide Levels and to correlate the C-peptide levels to the complications of diabetes mellitus. This prospective study was done on 50 lean diabetic patients who were selected based on BMI.(<18.5kg/m2.). Inclusion Criteria: Diabetic patients whose Body Mass Index (BMI) is <18.5 kg/m2. Patients from age group of 30-70 years. C-peptide is measured in all the patients by CLIA method (chemical linked immunosorbant assay), 2 hours after giving 100 grams of oral glucose (stimulated C-peptide levels). Exclusion Criteria: Diabetes patients with Body Mass Index >18.5 kg/m2, Patients with protein energy malnutrition from childhood, and Pregnant women, Patient suffering from any malignancies and Patients using long term steroid therapy (or) secondary cause of diabetes mellitus. The incidence of males affected were more than females Male : Female ratio (3:2), 36 (72%) of the patients are from middle socio economic status, 35 (70%) of patients had positive family history, 38 (76%) of patients were on OHAS, out of 50 patients, 43 (86%) patients had low c-peptide levels, and 7(14%) had normal c-peptide levels, 18 (36%) patients had Diabetic Retinopathy, 17 (34%) patients had Diabetic Nephropathy out of which 9 (18%) patients had micro albuminuria and 8 (16%) patients had macro albuminuria,18 (36%) patients had Diabetic Neuropathy. In this study most of the lean diabetics (43/50) patients had a low C-peptide levels, poorly controlled diabetes mellitus and diabetic Complications. These patients eventually required insulin for their control of diabetes.