To investigate the prevalence of true combined endodontic-periodontal lesions in adult patients with diabetes and compare them with normal patients of the same age group. Materials and Methods:A cross-sectional study was conducted on a sample of 152 patients that was divided into two equal groups: Group A (diabetic group) and Group B (control group). Clinical and radiographic examination were done to accurately detect the true combined endodontic-periodontal lesions. Data were then collected and statistically analyzed. Results: True combined endodontic periodontal lesions in group A were significantly higher than group B with percentage of 13.15% and 6.58% respectively (p<0.05). Males had significantly higher true combined endodontic periodontal lesions than females in both groups (p<0.05). In group A about 70% of these lesions were in patients below 60 years while in group B about 80% of the patients with combined endodontic periodontal lesions were above 60 years. Conclusion: Diabetics have a higher prevalence of true combined endodontic periodontal lesions and appears to be more prevalent in males than females. Age can be also and influencing factor in the development of these lesions.