The frequency and effects of hereditary diseases of consanguineous marriage (CM) in 210 965 Han nationality people in different districts of Zhejiang province were investigated. Of 44342 couples in three different regions, 951 (2.14%) were proved to be of CM. The frequency of CM was 3.02% in Mi as against 1.13% in Pi and 2.50% in Ii. There were 19 types and 81.07% of CM occurred in the first cousins, of which 0.95% occurred in semisiblings. The frequency of hereditary disease and the mortality before the age of 20 in the offsprings of CM were significantly higher than those of non-CM (P < 0.01). 104 types of hereditary disease and congenital malformation were found. The frequency of mental retardation was 2/1000. There were 197 high risk families which contained 19-types of hereditary disease.