Variations in trihalomethane (THM) levels during two-day period in the advanced treated water which had passed through a building's plumbing system (stored water) were compared with those in the advanced treated water delivered directly by the distribution system. THM levels in both the water were relatively uniform. The water quality deterioration measured by the increased THMs was apparently observed during the storage of the water in the plumbing system, even though the advanced treatment could produce high-quality water. Annual changes in THM in the stored water were also examined. Increasing air temperature increased THMs levels except for bromoform.