Adenomatous polyps are neoplasms that may progress to colorectal cancer. The role of diet and other lifestyle habits in their etiology is now being elucidated. The aim of this study was to evaluate effects of nutritional habits, weight and weight gain, tobacco smoking, and physical activity in adenoma etiology. A quantified dietary history questionnaire was designed to evaluate long-term dietary habits in addition to more recent ones. The study population comprised 196 adenoma patients and matched asymptomatic, screened controls. Statistical analysis used multivariate conditional logistic models, adjusting for total energy intake and physical activity. Odds ratios (ORs) and 95% confidence intervals (CIs) for adenoma associated with highest versus lowest tertiles of mean daily intake were as follows: for energy, OR 3.7 and CI 2.1-6.7; for animal fat, OR 2.4 and CI 1.2-4.7; for tobacco smoking, OR 3.1 and CI 1.1-2.8; and for weight gain, OR 2.2 and CI 1.2-4.1 (P for linear trend for all, < or = 0.01). Significant negative associations were found with intake of total carbohydrates (OR, 0.3; CI, 0.1-0.7) and fluids (OR, 0.4; CI, 0.2-0.8) (P for both < 0.01) as well as for physical activity (OR, 0.6; CI, 0.3-0.9; P = 0.03). Increased risk for adenoma was observed with decreased intake of carotene (OR, 0.6; CI, 0.3-1.0; P = 0.06), vitamin E (OR, 0.6; CI, 0.3-1.0; P = 0.07), and dietary fiber (OR, 0.6; CI, 0.3-1.3; not significant). The OR of interaction between water and dietary fiber was significant (OR, 0.7; CI, 0.6-0.9; P = 0.01), suggesting a synergistic protective effect. Specific dietary and lifestyle habits were identified as independent factors associated with colorectal adenomas; of special interest is the interaction between water and fiber intake. Avoiding these factors might delay or prevent neoplasia.