Of 138 grandchildren of patients, four had pyloric stenosis confirmed at Ramstedt's operation. For the four categories of grandchildren of male index patients the proportion affected was 0/27, 1/17, 1/41, and 0/35 for sons' sons, sons' daughters, daughters' sons, and daughters' daughters, respectively. For the four categories of grandchildren of female index patients the proportion affected was 0/5, 1/3, 0/5, and 1/5, respectively. Though numbers are small certain findings are noteworthy. The relatively high risk to the relatives of female index patients appears even more marked in the grandchildren than the children. The grandchildren of male index patients show the sharp decrease in proportion affected that one would expect with a multifactorial threshold model, but of the few grandchildren born so far to female index patients, as many are affected as the children. In only one of the four instances was the affected grandchild born to an affected child. There is no support for the hypothesis that the higher risk to the children of female index patients is the result of a direct maternal effect on the developing fetus.