In this nested case-control study, 8,006 American Japanese men were examined and interviewed with a dietary questionnaire from 1965 to 1968. After a follow-up period of over 16 years, 102 colon and 60 rectal cancer incident cases were identified. Dietary data from these patients and from 361 cancer-free controls were analyzed for intake of dietary fiber (DF), vitamins, minerals, macronutrients, and selected food groups. We found a significant (p = 0.042) negative association of DF and colon cancer risk among low fat intake men (<6I g/d). In this subgroup, the men consuming <7.5 g/d of DF had an adjusted relative risk (RR) for colon cancer of 2.28 (95% CI 0.93-5.60), compared to those consuming ⩾ 14.8 g/d of DF. We also observed (among the complete group of subjects) a significant (p = 0.011) negative association between vitamin C intake and the risk of colon cancer. Men in the lowest quintile of vitamin C intake (<37 mg/d) had an adjusted colon cancer RR of 1.87 (95% CI 1.03-3.37), compared to men in the highest quintile (⩾ 160 mg/d). We view these dietary associations with colon cancer risk with caution. There were no other significant associations of dietary variables with colon cancer risk. Also, there were no significant associations between intake levels of DF, micronutrients, or food groups and rectal cancer risk.