We investigated twenty-one insulin-using patients, who had all been labelled as having insulin dependent diabetes mellitus (IDDM) or type 1 diabetes. Physicians have been erroneously using the term IDDM loosely to include all diabetics on insulin. The clinical criteria of the National Diabetes Data Group/WHO were used to reclassify these patients. Only thirteen were found to have IDDM and eight non-insulin dependent diabetes mellitus (NIDDM). Using fasting C-peptide values, only five of the thirteen with clinical IDDM truly had IDDM, the others might have maturity onset diabetes of the young (MODY) or diabetes in the young. Of the eight with clinical NIDDM seven had normal to high C-peptide values; the lone patient with low C-peptide values had diabetes diagnosed at age 64 years. We conclude that the clinical classification of diabetes mellitus may be inaccurate and that C-peptide evaluation improves the accuracy of the classification.