Background: Diabetes is a common disease. There are some reports that indicate higher prevalence of coronary artery disease (CAD) in diabetic than nondiabetic patients, thus evaluating CAD in such patients is of prime importance. Objectives:The aim of present study was to compare the extent of CAD in the diabetic and nondiabetic patients. Patients and Methods:In this case-control study 65 diabetic patients (case group) were compared with 145 nondiabetic patients (control group) based on severity of coronary artery stenosis at heart center of Mostafa Khomeini hospital in Tehran (Iran) in 2007. Both groups were matched for age, sex and risk factors. Coronary artery status was evaluated by coronary angiography followed by analysis of data using statistical methods. Results: Based on data found in our study, 93.8% of diabetic and 83.4% of nondiabetic patients were shown to suffer coronary artery stenosis. Severe involvement (grade 3VD) of coronary artery stenosis was observed in 44.6% and 28.8% of diabetic and nondiabetic patients, respectively. Statistically, a significant difference was found between two groups regarding the rate and severity of CAD (P < 0.05). The occurrence of coronary stenosis was higher in females among both groups yet the difference insignificant, statistically (P > 0.05). Conclusions: Our findings revealed that the severity of coronary artery stenosis is common in diabetic patients compared to nondiabetics. Early diagnosis and treatment of CAD in diabetic patients is recommended.