Magnesium maintains genomic stability and is an essential cofactor for DNA synthesis and repair. Magnesium intake has been reported to be inversely associated with colorectal cancer (CRC) risk in Western populations. This study examined the association between dietary intake of magnesium and CRC risk in Japanese men and women aged 45-74 y. Data from 40,830 men and 46,287 women, at the 5-y follow-up of the Japan Public Health Center-based Prospective Study, who responded to a 138-item FFQ were used in this analysis. A total of 689 and 440 CRC events were observed during the mean follow-up of 7.9 and 8.3 y for men and women, respectively. When adjusted for potential confounders, the hazard ratio and 95% CI in the highest quintile of magnesium intake compared with the lowest quintile in men were 0.65 (95% CI, 0.40-1.03) for CRC (P-trend = 0.04), 0.48 (95% CI, 0.26-0.89) for colon cancer (P-trend = 0.01), and 0.97 (95% CI, 0.47-2.02) for rectal cancer (P-trend = 0.93). Borderline inverse associations were also observed in men who consumed alcohol regularly (P-trend = 0.07) or had a BMI <25 kg/m(2) (P-trend = 0.06). There were similar inverse associations for invasive colon cancer and distal colon cancer. There were no significant associations between magnesium intake and cancer risk in women. Higher dietary intake of magnesium may decrease the risk of CRC in Japanese men.