Summary and ConsclusionsOur study illustrates the variation in the severity of familial polyposis and the unpredictability of its course as found in members of related and unrelated families. After the onset of polyposis, the symptoms which emerge are dependent upon the rate of increase in the number and/or size of the adenomas and/or development of adenocarcinoma.Documented cases of familial diffuse polyposis which have had their onset (appearance of polyps) after the second decade of life are rare. In our studies of a limited number of affected families, onset of the disease in the first two decades of life was common; while of those who failed by examination to show the onset of polyposis in the first and second decades of life and who had follow‐up studies extending into the third and fourth decades, none was found to manifest the disease.In all reported studies, including ours, too few members of affected families have been examined throughout consecutive decades of life, especially during the second, third, and fourth, to determine the time of onset of familial polyposis. Beyond the “teen‐age,” each additional decade of freedom from the onset of familial polyposis greatly increases the probability that a member of an affected family has escaped the inherited disease. An affected member cannot be assured that he is free of the risk of recurrence of polyps and/or development of adenocarcinoma so long as he retains any remnant of the large intestine.