No specific pathognomonic tissue changes establish death from water intoxication, which makes a postmortem diagnosis of water intoxication difficult. We report an autopsy case of fatal water intoxication in which the postmortem computed tomography (PMCT) examination showed findings suggestive of antemortem excessive water drinking, such as diluted intestinal contents and hemodilution. A 52-year-old man who had schizophrenia was found dead in his house. He frequently complained of polydipsia, and drank a large amount of water and vomited before his death. PMCT findings showed distention of the duodenum with low radiodensity content (7 HU). In addition, the right atrium showed a low radiodensity content (46 HU). Autopsy results showed moderate congestion in all the organs, a large amount of light-brownish liquid in the duodenum and upper jejunum, and aspiration pneumonia. Electrolyte concentrations of sodium in the vitreous humor of the right and left eyes were 105 and 107 mEq/L, respectively, and neither drugs nor alcohol was detected in the blood. On the basis of the autopsy and biochemical findings, we determined water intoxication as the cause of death. An evaluation of PMCT findings of cases of various causes of death indicated that the radiodensities of the right atrium and duodenal content in our case were lower than those of the cases of various causes of death, except for fresh water drowning. The present case showed that PMCT provides supplementary findings regarding antemortem excessive water drinking. In addition, PMCT may provide findings to consider death from water intoxication before autopsy.