The incidence of colorectal cancer is rapidly increasing in Japan. This trend has been suggested to be caused by an increasing fat intake as a result of the Westernized diet among Japanese. We investigated whether dietary instruction optimizing the fat energy ratio suppresses the recurrence of colorectal tumors. The subjects, 373 men and women, were the participants in a randomized clinical trial of colorectal cancer prophylaxis. At entry, each participant completed a 3-consecutive-day food record on which dietary instruction was given to restrict fat energy ratio to 18-22%. Data obtained before and after the intervention were examined by cohort analysis. The primary endpoint was the presence or absence of colorectal tumor(s) at colonoscopy after 4 yr. Unexpectedly, the recurrence of tumor increased as the subjects reduced their fat intake. The lowest tumor recurrence among the men was observed in the group with 23.8-26.4% fat energy ratio after the intervention. Furthermore, in men, the risk of tumors decreased significantly as the intake of linoleic acids per body weight increased. For women, similar trends were observed. These results suggest that extreme fat restriction is highly likely to promote the recurrence of colorectal tumors, which may be partly attributable to linoleic acid deficiency.