OBJECTIVE The main purpose of this study is to emphasize the relevance of exfoliative cytology as an additional tool to aid in the diagnosis of diabetes mellitus. METHODS This is a comparative cross-sectional study. Oral smears were obtained from 10 diabetic patients and 10 healthy individuals. These smears were stained with Papanicolaou stain. The nuclear (NA) and cytoplasmic (CA) areas of 20 integral cells predominant in the buccal mucosa were measured using the Leica Qwin Version 2.1 image analysis system (LEICA GMBH GERMANY).The cytoplasmic/nuclear ratio (C/N) was then calculated. For comparing cytomorphometric parameters (NA, CA & C/N ratio) the Mann-Whitney test was used. Significance was set at P < or = 0.05. RESULTS The morphologic alterations seen in buccal mucosal epithelial cells of the diabetic group were nuclear enlargement, karyorrhexis, binucleation and infiltration of polymorphonuclear leukocytes. The NA was significantly higher (P < 0.05) in the diabetic group. The CA between these two groups were not significantly different (P > 0.05). The C/N mean was significantly lower (P < 0.05) in the diabetic group. CONCLUSIONS Exfoliative cytology is useful as an additional tool to aid in the diagnosis of diabetes mellitus.