It is well known that classification of diabetes in the group of patients aged 25-55 years, particularly not obese, is a difficult clinical problem. The aim of the study was to estimate clinical status of the investigated group and evaluate biochemical features based on positive titers of humoral immunological markers in the development of diabetes mellitus. We studied 31 non obese, newly diagnosed patients (mean BMI < 26 kg/m2), mean age--41.6 +/- 11.1 years, mean duration of diabetes mellitus was 11.4 +/- 21.8 months. Nearly 50% of the patients aged 25-55 years showed presence of humoral immunological markers, which suggests high prevalence of autoimmunological diabetes. These patients have a lower reserve of endogen insulin secretion. The evaluation at both B-cell secretory capacity and markers of autoimmunity is recommended to provide a proper classification.