Both diabetes mellitus and major depression are public health concerns, and the co-occurrence of these illnesses is highly frequent. Acting as a potential risk factor, hyperglycemia might facilitate the manifestation of depression in patients genetically predisposed to affective disorders. In the present study, candidate polymorphisms of the serotonin transporter, the tryptophan hydroxylase 2 (TPH2) genes, as well as of the brain-derived neurotrophic factor BDNF, and the P2RX7 purinergic receptor genes were analyzed in Hungarian diabetic population. We assumed that genetic influence would be stronger on depressive symptoms in the "poor glycemic control" group (PC: HbA1C>7%) compared to the "good glycemic control" group (GC: HbA1C<or=7%).