Objectives: To determine the penile size in adult Egyptians suffering from diabetes mellitus (detecting the onset, duration, and treatment for diabetes) and normal adults, and to clarify the relationship between: fully stretched, flaccid and erectile penile measurements in normal subjects and diabetic patients. Methods: This study included 200 adult Egyptians presenting to Kasr El-Aini outpatient clinic. Two groups of patients were included in this research work. Group one, normal adults (100), and group two diabetic patients (100). Penile length was measured using a ruler, in the flaccid, fully stretched and erect states in both groups. All penile measurements were performed by the same physician.Results: The diabetics group included 100 males. Their age ranged from 20 to 65 years (mean±SD 54.46±7.608). Waist circumference ranged from 60 to 160 cm (mean ±SD 121.22±17.196). Their flaccid penile length ranged from 6 to 13 cm (mean± SD 8.45±1.645), the stretched ranged from 8 to 17cm (mean± SD 11.00±1.834) and the erected ranged from 9 to 17 cm (mean±SD 12.17±1.640). The control group included 100 males. Their age ranged from 20 to 65 years (mean±SD 41.00 ±12.633). Their Waist circumference ranged from 95 to 160 cm (mean±SD 122.95±15.244). Their flaccid penile length ranged from 7 to 15 cm (mean± SD 9.99±1.487), the stretched ranged from 8 to 19 cm (mean±SD 12.68±1.874) and the erected ranged from 11 to 19 cm (mean±SD 14.30±1.860). By comparing the flaccid, stretched and erected penile length in patients and controls, we found that there was statistically significant decrease in length in patients than in the controls (p<0.05). Also, by comparing the flaccid, stretched and erected penile length in patients with controlled diabetes and those with uncontrolled diabetes, we found that there was statistically significant decrease in length in uncontrolled group than in the controlled group (p<0.05). In addition, we found a statistically significant negative correlation between HBA1C levels when compared with stretched penile length in diabetic patients, and a non-significant correlation when compared with flaccid and erected penile length. Conclusion: Our study shows significant correlation between relatively shorter penile lengths and diabetes mellitus, that is relevant with degrees of control, and which is irrelevant with the specific type of diabetes mellitus, its medications, and its duration.