We undertook a study to provide a quantitative basis to the clinical observation of an unexpected high frequency of atrial fibrillation in patients with colorectal cancer. In our study we considered a total of 1463 patients admitted during a period of 12 years (1987-1998) to the Department of Surgery of our Hospital for surgical treatment of colorectal cancer (case group: mean age 66 years) or non-neoplastic diseases including inguinal hernia, cholelithiasis, varicose veins and hemorrhoids (control group: mean age 65.6 years). We found a three times higher likelihood of having atrial fibrillation in patients with first diagnosis of colorectal cancer compared to controls. Logistic analysis was used to exclude an effect of age. Our study suggests that atrial fibrillation could be considered a non metastatic effect of colorectal cancer. Several pathophysiological mechanisms could explain this observation. A recent study, which reports a significant elevation of C-reactive protein in patients with atrial fibrillation, suggests that this arrhythmia could be promoted by a systemic inflammatory state. In conclusion, atrial fibrillation could be considered a pathological condition not only dependent to organic heart diseases; further studies are needed to evaluate the real impact of systemic inflammatory state or other non-cardiovascular diseases on the excess of mortality in the patients with atrial fibrillation.