Oral lichen planus (OLP) is a chronic inflammatory mucocutaneous disease, predominantly seen in middle-aged female patients. Histopathologically, OLP includes parakeratosis with T-lymphocyte subepithelial infiltration. In etiology of OLP, chronically liver and biliary diseases, stress-induced hypertension, inheritance and most commonly carbohydrate metabolism disorders including diabetes mellitus are mentioned. Considering the autoimmune nature of the illness and the role of the HLA DR 3/4 antigens in development of diabetes mellitus we hypothesized that the same antigens could play role in the development of OLP. We examined 50 unrelated Croatian patients (5 males) from Institute of Oral Pathology, Faculty of Dental Medicine, Zagreb, with clinical and histopathological diagnosis of OLP. They were all tested for HLA DR and compared with 1089 Croatian controls that underwent the tissue standardization in fathership cases. OLP patients had blood sugar level controlled, and those with HLA DR 3/4 and those with blood sugar level higher then 5.6 mmol/l additionally underwent standard oral glucose tolerance test. Carbohydrate metabolism disorder was found in 13 OLP patients (26%) while 10 of them (20%) had diabetes mellitus type I. This finding is 400 times more frequent than is appearance of diabetes in healthy controls (0.05%) in Croatia.