The metabolic syndrome, a cluster of metabolic abnormalities linked to insulin resistance, has attracted much interest as a risk factor for cardiovascular disease and type 2 diabetes. Hyperinsulinemia is also a postulated biological risk factor for colorectal carcinogenesis. We therefore here examined the relation between the metabolic syndrome and colorectal adenoma development. The study subjects were 756 cases of colorectal adenoma and 1751 controls with no polyps who underwent total colonoscopy during the period January 1995 to March 2002 at two Self Defense Forces (SDF) hospitals in Japan. The metabolic syndrome was defined with reference to abdominal obesity in combination with any two of the following conditions: elevated triglycerides (150 mg/dL); lowered HDL cholesterol (<40 mg/dL); elevated blood pressure (systolic blood pressure 130 mmHg and/or diastolic blood pressure 85 mmHg); and raised fasting glucose (110 mg/dL). Abdominal obesity was defined as a waist circumference of 85 cm or more(Japanese criterion) or 90 cm (Asian criterion). Statistical adjustment was made for age, hospital, and rank in the SDF. The metabolic syndrome was found to be associated with a moderately increased risk of colorectal adenomas whether either of the Japanese and Asian criteria was used; adjusted odds ratios with the Japanese and Asian criteria were 1.38 (95% confidence interval [CI] 1.13-1.69) and 1.48 (95% CI 1.13-1.93), respectively. Increased risk was more evident for proximal than distal colon or rectal adenomas, and was almost exclusively observed for large lesions (5 mm in diameter). Thus the metabolic syndrome appears to be an important entity with regard to the prevention of colorectal cancer, as well as cardiovascular disease and type 2 diabetes.