Aspirin has been associated to a reduced risk of colorectal, and possibly of a few other common cancers. Epidemiological studies on aspirin and cancer risk published up to December 2005 have been reviewed, and pooled relative risks (RR) for several cancers have been provided. Besides a reduction in the risk of cancer of the colorectum (RR = 0.71, 95% confidence interval, CI: 0.67-0.75), there is evidence-although more limited, and mainly from case-control studies-that aspirin has a favourable effect on cancers of the esophagus (RR = 0.72, 95% CI: 0.62-0.84), stomach (RR = 0.84, 95% CI: 0.76-0.93), breast (RR = 0.91, 95% CI: 0.88-0.95), ovary (RR = 0.89, 95% CI: 0.78-1.02) and lung (RR = 0.94, 95% CI: 0.89-1.00). The role of aspirin on other cancers, such as pancreatic, prostate, bladder cancer, and non-Hodgkins' lymphomas is less clear, and an increase of risk has been suggested for kidney cancer. For most cancer sites, however, significant heterogeneity between studies, and particularly across study design, was found, with a reduction in risk generally stronger in case-control than in cohort studies. Further, notwithstanding the large amount of epidemiological evidence, substantial uncertainties remain about the proper aspirin dose and duration of treatment.