Statistical and clinical studies were conducted on familial occurrence rates of hepatoma and liver cirrhosis in adults with 37 cases of primary hepatoma and 246 cases of liver cirrhosis admitted to our clinic during the 9-year period of 1958 to 1966. As a result there were found 29 families showing familial occurrence of hepatoma or liver cirrhosis. The results of the study are briefly summarized as follows. 1) The incidence of hepatoma or liver cirrhosis among the relatives of 283 subjects was 1.9% in brother-sister of the patient; 3.9% in the parent; and 0.44% in grandfather-grandmother. These rates were significantly higher than those of the control hepatitis group. 2) The onset rates in brother-sister of index cases (under whom one of the affected individuals in the lowest ranked generation was designated) among 29 pedigrees of familial occurrence proved to be 32% (12/38) in 8 families of the brother-sister group; 10% (7/70) in 16 families of the parent-child groups (of them 1/29=3.4% in 7 families of the father-child group, 6/22=27% in 6 families of mother-child group, and 0/19 in 3 families of parents-child group). It was 17% (2/12) in the 3 families of 3 consecutive generations (grandparent, mother and their descendants), and in two families of the grandmother as mother's side-grandchild group it was 0/8 (individual). In addition, in the parents of the index cases there was no cousin-to-cousin marriage. 3) In the same pedigree there could be detected some cases showing similar findings of onset age, type of disease, timelapse pathohistological findings, despite the difference in their modes of living and history. Further, among the patients investigated there were some who took so rapid course from chronic hepatitis of active type or from submassive hepatic necrosis to liver cirrhosis that they could not receive sufficient benefits of steroid treatment or those who died precipitously because of fulminant disease. 4) In these 29 pedigrees studied, there were observed epidemic nature of hepatitis and drinking as exogenous factors and the propensity to diabetes as inherent cause. Judging from such factors as hepatitis epidemic in the residential district of patients, distance between the residence of patients, the difference in the onset age, and hepatitis history among relatives, it has been assumed that rather than emphasizing epidemic nature of hepatitis in the familial occurrence, a greater emphasis should be placed on the propensity of the patient to hepatitis and on the predisposition to shift to liver cirrhosis. As for the alcohol drinking, those heavy drinkers showed a higher incidence than liver cirrhosis patients showing no familial trend, but there was obserbed onset of hepatoma or liver cirrhosis even in females and males without history of drinking, and in 29 pedigrees the drinking that proved to be a common cause in the family was limited within 4 pedigrees. As for the causative factor of diabetes, subjects related by blood having diabetes gave a higher incidence than liver cirrhosis patients not showing familial trend, but there was not a single pedigree having over 2 affected individuals complicated with diabetes in the same family. In other words, these exogenous and inherent causes couldn't be taken as the principal causes of familial occurrence. 5) In the mother-child group of familial occurrence the rate of incidence was not only higher in child but also there was a trend of the patient to be succeeded by other affected siblings in the order of their birth. In our Au antigen tests conducted with 2 pedigrees, the rest result was positive in the mother (inflicted) and daughter (not inflicted), while inflicted father gave a positive result but the child was negative. 6) These results suggest that there were genetic factors involved in familial occurrence of hepatoma or liver cirrhosis, and in the mother and child group there was indicated hepatitic viral intrafamilial infection in addition.