A prospective case control study was conducted in 859 diabetic probands and 1059 nondiabetic controls. These were interviewed for history of diabetes mellitus among their first degree relatives. The protocol also included second and third degree relatives. The prevalence of diabetes mellitus in the first degree relatives of cases and controls showed a statistically significant difference (Z = 6.7564 and P = 0.00000). Similarly, the prevalence of diabetes among the second and third degree relatives of cases and controls differed significantly (Z = 2.74, P = 0.006). There were 445 non-insulin dependent diabetes mellitus (NIDDM) and 414 insulin dependent diabetes mellitus (IDDM) in the diabetic probands. There was more family history of diabetes mellitus among the first degree relatives of NIDDM than IDDM and the difference is statistically significant (Z = 4.076, P = 0000). The second and third degree relatives of NIDDM and IDDM also differed significantly (Z = 4.05, P = 00005). This study indicates that heredity plays an important role in the genesis of diabetes mellitus in Ethiopian diabetics.