Monson, R. R. (Harvard School of Public Health, Boston, Mass. 02115). Familial factors in peptic ulcer. I. The occurrence of ulcer in relatives. Amer. J. Epid., 1970, 97: 453–459—The occurrence of peptic ulcer in relatives of physicians with a history of ulcer has been compared to that in relatives of physicians without ulcer. About twice as many duodenal ulcers as expected occurred in relatives of those with duodenal ulcer, but the gastric ulcer experience in these relatives was no different than that expected. Also, the age-specific prevalence rates of peptic ulcer as estimated from a survey in physicians were used to calculate the expected numbers of ulcers in fathers and brothers of physicians with ulcer. These calculations resulted in a larger number of expected ulcers for brothers than obtained in comparison with brothers of controls; the expected number for fathers was the same using the two methods. This suggests that physicians with ulcer were not aware of all the ulcers that had occurred in their brothers. Evidence is presented that physicians without ulcer have somewhat less information about familial occurrence of ulcer than physicians with ulcer, especially with respect to deceased relatives. Relatives of physicians with gastric ulcer had an ulcer experience no different from that expected.