BACKGROUND: Type II diabetes mellitus (DM) is one of the leading chronic diseases in Qatar as well as worldwide. However, the risk factors for DM in Qatar and their prevalence are not well understood. We conducted a case-control study with the specific aim of estimating, based on data from outpatients with DM in Qatar (cases) and outpatient/inpatient controls, the association between demographic/lifestyle factors and DM. METHODS: A total of 459 patients with DM from Hamad General Hospital (HGH) outpatient adult diabetes clinics, and 342 control patients from various outpatient clinics and inpatient departments within Hamad Medical Corporation (HMC) (years 2006-2008), were recruited. The association between risk factors and DM was evaluated using bivariate and multivariable logistic regression analyses. In addition to odds ratios (OR) and 95% confidence intervals (95% CI), we estimated the population attributable risk fractions for the DM demographic/lifestyle risk factors. RESULTS: Qatari nationality was the strongest risk factor for DM (adjusted OR = 5.5; 95% CI = 3.5-8.6; p < 0.0001), followed by higher monthly income (defined as ≥ 3000 Qatari Riyals, adjusted OR = 5.1; 95% CI = 3.0-8.7; p < 0.0001), age >65 years (adjusted OR = 3.3; 95% CI = 0.9-11.4; p = 0.06), male gender (adjusted OR = 2.9; 95% CI = 1.8-4.8; p < 0.0001), obesity (BMI ≥ 30, adjusted OR = 2.2; 95% CI = 1.5-3.2; p < 0.0001), no college education (adjusted OR = 1.7; 95% CI = 1.2-2.6; p = 0.009), and no daily vigorous/moderate activity (adjusted OR = 1.5; 95% CI = 0.9-2.3; p = 0.12). Among Qatari nationals, obesity was found to be the main risk factor for DM (unadjusted OR = 3.0; 95% CI = 1.6-5.6; p < 0.0001), followed by no college education (unadjusted OR = 2.7; 95% CI = 1.5-5.1; p = 0.001), while consanguinity did not appear to play a major role in predicting DM (unadjusted OR = 1.5; 95% CI = 0.8-2.8; p = 0.21). Our findings further suggested that eliminating obesity and improving access to education may reduce DM cases by up to one third for the population at large (31.7% and 26.8%, respectively) and up to half (46.9% and 49.3%, respectively) for Qatari nationals. Promoting physical activity may reduce the burden of DM by up to 9.4% for the population at large and up to 17.3% for Qatari nationals. CONCLUSIONS: Demographic/lifestyle factors appear to be the main risk factors for the high DM levels observed in Qatar, with a contribution that outweighs that of genetic risk factors. While further evaluation of DM risk factors among the Qatari population (as opposed to the resident population) is important and of interest, these findings highlight the need to focus short-term DM interventions on addressing demographic/lifestyle risk factors to achieve substantial and timely declines in DM levels.