218 diabetic probands and 300 non-diabetic controls were interviewed for history of diabetes mellitus in their parents and siblings. The prevalence of diabetes mellitus among blood relatives of probands and controls differed significantly. The study also showed that NIDDM probands had more positive family history of diabetes than IDDM ones. The overall findings of this study indicated that heredity has an important role in the genesis of diabetes mellitus in Ethiopian diabetics.