Differentiating NIDDM-Y from IDDM in youth is a diagnostic challenge. Serum insulin levels at diagnosis may help differentiate between NIDDM-Y and IDDM if the level is elevated, but the serum insulin level is low or undetectable in 45% of patients with NIDDM-Y. Islet-specific antibodies may be present in serum at diagnosis, and ketosis or ketoacidosis may occur. For our patients, clinical features are most helpful in differentiating NIDDM-Y from IDDM and include ethnic background, age and gender at diagnosis (approximately 80% of First Nation patients from northern Manitoba are adolescent females), presence of obesity and acanthosis nigricans, lack of symptoms or weight loss, and strong family history of NIDDM.