Aim of the present study was to assess the prevalence of atrial fibrillation (AF) in patients with two different types of cancer. Recent epidemiologic and clinical studies support the hypothesis that AF is promoted and maintained by a broad spectrum of modulating factors. A total of 2,339 patients admitted to the Surgery Department of "Luigi Sacco Hospital, Milan," over the period 1987-2004 were eligible for the study. One thousand three hundred and seventeen patients were admitted consecutively with a first diagnosis of colorectal or breast cancer (cases). The remaining 1,022 were patients admitted to undergo non-neoplastic surgery (controls). Routine pre-surgery electrocardiogram available in patient charts was analysed by a cardiologist who was not aware of the present study to evaluate the presence of atrial fibrillation or other arrhythmias. Overall, AF was present in 3.6% cases and 1.6% controls. This corresponded to at least two times higher likelihood of having AF in cases compared to controls. Prevalence of AF increased with age both in cases and controls. Our study describes an increased prevalence of AF in two different types of cancer. Autonomic, endocrine, coagulation, and inflammatory alterations were previously described in both AF and cancer, and can provide the physiopathological basis to our clinical observation.