In the summer of 1980, the Washington Suburban Sanitary Commission (WSSC) put the Germantown elevated tank into operation. During the first several months of operation, the water level in the tank rarely exceeds an elevation of 3 m below the maximum water elevation for which it was designed. The article describes the investigation conducted by WSSC to determine the cause of this problem. The investigation utilized computer modeling techniques. Based on the results of the modeling, an intensive field investigation was begun. Eventually the field investigation found that a valve on a 900 mm pipe leading to the Germantown tank was unintentionally closed and a pressure relief valve close to the tank was set to open, releasing water from the distribution system at an elevation 2.1 m below the maximum water level in the tank. Both of these problems have since been remedied and the tank now fills to its maximum level. Includes figures.