Previous studies suggest that inflammation plays an important role in pathogenesis of diabetes. Cytokines may play positive or negative role in immunological reactions. Among many cytokines, interleukin 12 (IL-12) is known to be a strong pro-inflammatory cytokine. A total of 76 participants were enrolled in this study and classified in 4 groups: 23 diabetic patients with non-treated retinopathy, 17 diabetic patients with treated retinopathy, 12 diabetic patients without retinopathy, and 24 healthy control patients. Serum and aqueous humor samples were taken for the analysis of IL-12 concentration. There was a significant difference between the groups in IL-12 concentrations in the aqueous humor (χ2 (3,N=76)=27.137; p=5.5*10-6 ) with highest values measured in the non-treated diabetic retinopathy group (12,40 pg/mL). No significant differences in IL-12 serum concentrations between the groups were found (F=0.405, P=0.750). Correlation analysis of IL-12 concentrations in the serum and aqueous humor showed association between the two variables only in non-diabetic patients (P=0.003). To the best of our knowledge, this is the first study to show a significantly higher concentration of pro-inflammatory cytokine IL-12 in the aqueous humor of non-treated diabetic retinopathy patients in comparison with diabetic patients treated for retinopathy, diabetic patients without retinopathy or with healthy individuals. Because the serum levels of IL-12 did not differ considerably between the studied groups, it is possible that this is due to its local production and secretion.