The possible association between the risk of liver cancer and the levels of magnesium in drinking water from municipal supplies was investigated in a matched cancer case-control study in Taiwan. All eligible liver-cancer deaths (9166 cases) of Taiwan residents from 1994 through 1998 were compared with deaths from other causes (9166 controls). The levels of magnesium in the drinking water of these residents were also determined. Data on magnesium levels in drinking water throughout Taiwan were obtained from the Taiwan Water Supply Corporation (TWSC). The control group consisted of people who died from other causes, and the controls were pair-matched to the cases by year of birth and death. The adjusted odd ratios for the relationship between magnesium levels in drinking water and liver cancer were not statistically significant. Confounding variables, hepatitis B and C infection for example, are probably most likely in the genesis of liver cancer.