Evidence of a protective role of aspirin on the risk of colorectal and other common cancers has been building up since the end of the 1980s. There are now more than 15 epidemiological (case-control and cohort) studies indicating that long-term use of aspirin is associated with a reduced risk of colorectal cancer. The overall relative risk (RR) estimate for regular aspirin users was 0.71 (95% confidence interval (CI) 0.66-0.77) from case-control studies, and 0.84 (95% CI 0.72-0.98) from cohort studies. A recent meta-analysis reported a RR of breast cancer for aspirin use of 0.70 (95% CI 0.61-0.81) in case-control studies, and of 0.79 (95% CI 0.59-1.06) in cohort studies. Furthermore, various epidemiological studies have suggested that aspirin use might have a favourable effect on ovarian cancer as well: the overall RR estimate was 0.82 (95% CI 0.69-0.99), although the evidence is too limited to permit firm conclusions. Data are more scanty, though in the same direction, for other neoplasms, including in particular stomach and oesophageal cancer.