CONTEXT: Folate, vitamin B(6), and vitamin B(12) are thought to play an important role in cancer prevention. OBJECTIVE: To evaluate the effect of combined folic acid, vitamin B(6), and vitamin B(12) treatment on cancer risk in women at high risk for cardiovascular disease. DESIGN, SETTING, AND PARTICIPANTS: In the Women's Antioxidant and Folic Acid Cardiovascular Study, 5442 US female health professionals aged 42 years or older, with preexisting cardiovascular disease or 3 or more coronary risk factors, were randomly assigned to receive either a daily combination of folic acid, vitamin B(6), and vitamin B(12) or a matching placebo. They were treated for 7.3 years from April 1998 through July 31, 2005. INTERVENTION: Daily supplementation of a combination of 2.5 mg of folic acid, 50 mg of vitamin B(6), and 1 mg of vitamin B(12) (n = 2721) or placebo (n = 2721). MAIN OUTCOME MEASURES: Confirmed newly diagnosed total invasive cancer or breast cancer. RESULTS: A total of 379 women developed invasive cancer (187 in the active treatment group and 192 in the placebo group). Compared with placebo, women receiving the active treatment had similar risk of developing total invasive cancer (101.1/10,000 person-years for the active treatment group vs 104.3/10,000 person-years for placebo group; hazard ratio [HR], 0.97; 95% confidence interval [CI], 0.79-1.18; P = .75), breast cancer (37.8/10,000 person-years vs 45.6/10,000 person-years, respectively; HR, 0.83; 95% CI, 0.60-1.14; P = .24), or any cancer death (24.6/10,000 person-years vs 30.1/10,000 person-years, respectively; HR, 0.82; 95% CI, 0.56-1.21; P = .32). CONCLUSION: Combined folic acid, vitamin B(6), and vitamin B(12) treatment had no significant effect on overall risk of total invasive cancer or breast cancer among women during the folic acid fortification era. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT00000541.