Background : Reduced methylation of DNA may contribute to loss of the normal controls on proto-oncogene expression. In humans, hypomethylation of DNA has been observed in colorectal cancers and in their adenomatous polyp precursors. Accumulation of DNA methylation abnormalities, observed during progression of human colorectal neoplasia, may be influenced by certain dietary factors. The apparent protective effect of fresh fruits and vegetables, the major folate sources, on colorectal cancer incidence suggests that a methyldeficient diet contributes to occurrence of this malignancy. Low dietary folate and methionine and high intake of alcohol may reduce levels of S -adeno-sylmethionine, which is required for DNA methylation. Purpose : To determine if dietary factors that may influence methyl availability are related to colorectal adenomas, we prospectively examined the association of folate, methionine, and alcohol intakes and risk of colorectal adenoma. Methods : We assessed dietary intake for a 1-year period for women of the Nurses' Health Study, started in 1976, and for men of the Health Professionals Follow-up Study, started in 1986—using a semiquantitative food frequency questionnaire. Adenomatous polyps of the left colon or rectum were diagnosed in 564 of 15984 women who had had an endoscopy between 1980 and 1990 and in 331 of 9490 men who had undergone an endoscopy between 1986 and 1990. Results : High dietary folate was inversely associated with risk of colorectal adenoma in women (multivariate relative risk [RR] = 0.66; 95% confidence interval [CI] = 0.46–0.95 between high and low quintiles of intake) and in men (RR = 0.63; 95% CI = 0.41–0.98) after adjusting for age, family history, indications for endoscopy, history of previous endoscopy, total energy intake, saturated fat intake, dietary fiber, and body mass index. Relative to nondrinkers, drinkers of more than 30 g of alcohol daily (about two drinks) had an elevated risk of adenoma (in women, RR = 1.84, 95% CI = 1.19–2.86; in men, RR = 1.64, 95% CI = 0.92–2.93). Methionine intake was inversely associated with risk of adenomas 1 cm or larger (RR = 0.62; 95% CI = 0.46–0.85, combining men and women). Conclusions : Folate, alcohol, and methionine could influence methyl group availability, and a methyl-deficient diet may be linked to early stages of colorectal neoplasia. A dietary pattern that increases methyl availability could reduce incidence of colorectal cancer. Implications : These data support efforts to increase dietary folate in segments of the population having diets with low intakes of this nutrient. [J Natl Cancer Inst 85:875–884, 1993]