During the last 15 years, seventeen patients with polypoid lesions or carcinoma in the colon or rectum were repeatedly examined with barium enemas. The doubling time of these colorectal tumors was calculated according to Spratt's theory of the growth rate of neoplasms. The doubling times of 6 carcinomas varied from 92.4 to 1032.2 days with those of early carcinomas being much shorter than those of advanced lesions. Advanced lesions of the proximal colon had a shorter doubling time than those of the distal colon. The doubling times of 6 adenomatous polyps ranged from 146.5 to 398.5 days, thus coinciding with the time interval of the carcinomas. However, 5 other adenomatous polyps did not grow during the follow-up period which ranged from 420 to 816 days. There was no definite correlation between the doubling time and the histology of the tumors. The difference in the doubling time of carcinomas correlated more with location than the histological grade of malignancy.