BACKGROUND: Only limited information and understanding are available on the potential relation between oral contraceptive use and the risk of colorectal cancer. Further data on the issue are therefore useful and may help informed choice of contraception. METHODS AND RESULTS: Data were derived from a case-control study of colorectal cancer conducted between 1992 and 2001 in the Swiss Canton of Vaud, including 131 women with colorectal cancer and 373 controls admitted in the same hospital as the cases with diagnosis of acute, non-neoplastic disease, unrelated to long-term modification of diet. Oral contraceptive use was reported by 11% of cases versus 17% of controls, corresponding to multivariate odds ratio of 0.8 (95% CI: 0.4-1.7). The odds ratio was non-significantly below unity across strata of duration, time since first and last oral contraceptive use. CONCLUSION: These findings add further evidence on a possible inverse relation between oral contraceptive and colorectal cancer risk. [authors]