Resection of the colon in patients with familial adenomatous polyposis frequently results in the regression of polyps in the remaining rectum, suggesting a reduction of cellular proliferation. These patients remain at risk of developing rectal cancer but whether this risk increases with time is uncertain. Since ornithine decarboxylase activity is associated with cellular proliferation, mucosal ornithine decarboxylase was measured in rectal biopsy specimens from patients with familial adenomatous polyposis after ileorectal anastomosis (n = 36) and from normal controls (n = 30). The relationship between ornithine decarboxylase activity, age, and time from surgery was also examined. Median ornithine decarboxylase activity in familial adenomatous polyposis patients after ileorectal anastomosis (186, interquartile range (IQR) 107-534 pmol/mg protein/h) was not different from that in control subjects (227, IQR 123-374, p = 0.6). When patients were divided into three equal groups according to age, however, younger patients (< 25 years) had significantly higher activity than both older age groups (p < 0.02). Similarly, when patients were stratified according to the time elapsed since surgery, those who had had surgery less than six years previously had a significantly higher ornithine decarboxylase activity than those in whom a longer time interval had elapsed since surgery (p = 0.02). These results indicate that after colon resection, ornithine decarboxylase activity in patients with familial adenomatous polyposis is similar to that in normal controls but seems to fall over time. This may explain the regression of rectal polyps after colonic resection in this disorder.