Incidence of neoplasms, benign and malignant, and of all other diseases was analyzed for 1,232 offspring of 551 breast cancer probands. Similar analysis was made for 1,487 offspring of 551 control index women matched by race, age, and year of death. Causes of death were verified by death certificates whereas morbidity was verified by hospital and/or physician's records. The average number of children born to the probands was 2.2 whereas such number was 27 for the control subjects. The over-all mortality rate was significantly lower among the proband offspring, which was accounted for by a lower risk particularly among those under 10 years of age. No generalized tendency of an increased risk of cancer was noted among the proband offspring. A marked increase was indicated in the incidence of breast cancer, benign breast tumors, and of benign skin tumors among the proband offspring. There might be causal relationship between benign and malignant breast tumors. Congenital malformations were more frequent among the proband offspring. There was no difference in neoplasms between those who were breast-fed and those who were not breast-fed. Both diabetes mellitus and circulatory diseases were less frequent among the proband offspring. Understanding of the biological mechanism of the association between breast and skin tumors and of an apparent antagonism between breast cancer and diabetes mellitus among those who are genetically similar might provide new dues for the etiology of breast cancer.