It has been reported that allergy and other diseases may be related to colorectal cancer risk. The aim of this study was to perform a systematic analysis using information about medical histories specifically to see if there was any relation between allergies or other medical conditions and colorectal cancer risk. A multicentric case-control study was conducted in six Italian areas between 1992 and 1996 on 1225 incident cases of colon cancer, 728 cases of rectal cancer and 4154 controls comparable with cases according to sex and age group, admitted for acute conditions to the same network of hospitals where cases had been identified. Unconditional logistic regression models including terms for sex, age, study centre, years of education, body mass index, physical activity, smoking, history of colorectal cancer in first-degree relatives and energy intake were used to estimate the odds ratios (OR) of colon and rectal cancer according to history of allergy and other selected diseases. The OR for history of allergy was 0.88 (95% confidence interval, CI, 0.67-1.14) for colon and 0.64 (95% CI, 0.44-0.92) for rectal cancer, and the inverse association was stronger when allergy was diagnosed at age 35 years or more, or less than 10 years before the cancer diagnosis. No clear pattern emerged in strata of age and sex. History of other selected diseases, including hypertension and cholelithiasis, was not related to colon or rectal cancer risk, though there was a moderate increase in the risk of colon cancer (OR = 1.18, 95% CI, 0.66-2.14) in patients with a history of intestinal polyps. This study lends support to the hypothesis that allergic individuals may be at a lower risk of developing colorectal cancer.