AIM:To analyze the origin of the anticipation phenomenon, which means earlier death in successive generation in familial adenomatous polyposis.METHODS:The study subjects were 2161 patients with familial adenomatous polyposis and their 7465 first degree relatives who were members of 750 families registered at our Polyposis Registry. The ages at death and cumulative mortality rates in theparent, the proband, and the child generations were compared for both all subjects and the patients alone.RESULTS:In the patients over 5 years of age, the mean age at death was 50.9 years for the parent, 42.3 years for the proband, and 33.3 years for the child generations, respectively a(c)(P < 0.001). The deceased rates in the three generations were 90.7%, 51.3% and 23.1% of the patients, respectively, and this difference was the main cause of the anticipation measured by parent-child paring method. The cumulative mortality rates for all subjects failed to show anticipation, however the cumulative mortality rates for the patients showed the anticipation. The anticipation phenomenon was shown by any parent-child pairing methods for the deceased patients. Other important causes of the anticipation were different proportion of causes of death between generations a(c)(P <0.001), and a low proportion of detected or deceased patients (P < 0.001) in the child generation.CONCLUSION:Anticipation in familial adenomatous polyposis may be caused by parent child paring methods as well as several intergenerational biases.