Previous epidemiologic studies have reported that a history of allergy is associated with reduced risk of colorectal cancer and other malignancies. We studied the association between allergy history and incident colorectal cancer (n = 410) prospectively in 21,292 Iowa women followed for 8 years. Allergy was defined from four self-reported questions about physician-diagnosed asthma (a), hay fever (b), eczema or allergy of the skin (c), and other allergic conditions (d). A history of any allergy was inversely associated with incident colorectal cancer: after multivariate adjustment, the hazard ratio (HR) was 0.74 [95% confidence interval (95% CI), 0.59-0.94]. Compared with women with no allergy, women reporting only one of the four types of allergy and women reporting two or more types had HRs of 0.75 (95% CI, 0.56-1.01) and 0.58 (95% CI, 0.37-0.90), respectively (P trend = 0.02). The inverse association persisted in analyses restricted to any type of nonasthmatic allergy (HR, 0.73; 95% CI, 0.56-0.95). HRs were similar for rectal and colon cancers as well as for colon subsites: proximal and distal (HRs for any allergy ranged from 0.63 to 0.78 across these end points). Allergy history, which may reflect enhanced immunosurveillance, is associated with a reduced risk of colorectal cancer.