BACKGROUND: The prevalence of type 2 diabetes in young adults and adolescents has increased in the last decade according to the increasing obese population. The aim of this study was to examine the clinical characteristics of patients diagnosed with diabetes mellitus before the age of 40 years as compared with patients diagnosed at older ages. METHODS: This was a cross-sectional, retrospective study using data from 350 diabetic patients who were diagnosed with diabetes in an outpatient setting between January 2005 and December 2007. Patients were diagnosed according to the criteria set forth by the American Diabetes Association. We examined the clinical characteristics and laboratory data of the patients through review of medical records and compared the early-onset diabetic patients (< 40 years old) and the usual-onset diabetic patients (> or = 40 years old). RESULTS: The frequency of early-onset diabetes and usual-onset diabetes were 31.1% (n=109) and 68.9% (n=241), respectively. The early-onset diabetic patients more often had a positive family history of diabetes; higher HbA1c, fasting glucose, and postprandial glucose levels; experienced typical symptoms more frequently; had microalbuminuria more frequently; and required insulin therapy as initial treatment more frequently as compared to usual-onset diabetic patients, and these differences were significant. Conversely, hypertension was significantly more common in the usual-onset diabetic patients. CONCLUSION: It could be concluded that we should control early onset diabetes more strictly to prevent its complication because early onset diabetic patients represented more severe hyperglycemia and had more prevalent microalbuminuria.