To discuss the inheritant mode of familial NIDDM. METHODS According to WHO criteria of DM, 100 NIDDM Probands with family history of DM were diagnosed and 100 persons were chosen at random for controls. The survey of DM was performed in both groups, including FBG, HbAlc, FINS. Some members had insulin release test. RESULTS The prevalance rates of DM in familial DM group were 26 times of the control group (34.3% and 1.3%). The prevalance rate of DM among first-degree relation was 18 times higher than that in general population (28.3% and 1.5%). The rate of diabetes in the siblings and in the off-springs was 44.4% and 9.7% respectively. The pedigree analyses showed that 83.9% affected families had one diabetic parent, one half siblings had DM, and there was a successive transmission of DM through at least three generations in sixteen large families. Besides, the incidence of DM was much higher in females than in males (40% and 28%). Among affected parents, diabetlic mothers were much more than diabetic fathers (50.8% and 27.6% P < 0.01). CONCLUSIONS Familial NIDDM had a familial aggregation. It was inherited in the manner of Mendelian autosomal dominant inheritance. The difference between the rate in DM mothers and fathers was probably due to unequal prevalance rate in females and males.