Background: It’s known that prisoners have worse physical and mental health conditions. Objective: This study aims to determine the prevalence of DM in the selected Turkish prisons. Methods: In this cross-sectional study, 722 volunteer from a total of 4000 prisoners in Ankara Sincan Prison Campus between March and June 2013, were enrolled. Results: Mean FBG level of whole study population was 101±28mg/dL. The mean age and BMI of the prisoners with DM was significantly higher when compared to the non-diabetics. We found the prevalence of DM as 3.8%. Despite the presence of 252 (FBG≥100 mg/dL) prisoners with impaired FBG, we could perform OGTT only in 12 prisoners. Three of them were diagnosed with impaired glucose tolerance, and one was diagnosed with overt diabetes. Conclusions: It may be estimated that there must be 4990 (3.8%) diabetic patients including 525 (0.4%) newly diagnosed ones among in all Turkish prisons. In our study, the most important handicap for determining DM prevalence in prisons is the tendency of the prisoners to hide their disease.