In the summers of 1993 and 1994, New York City experienced a series of positive total coliform and E. coli findings in its water distribution systems. A preliminary investigation indicated that several factors could have contributed to these occurrences. These factors included: the substantial increases in water usage during the hot summer months which disturbed the sediments in Hillview Reservoir; waterfowl in the reservoir; and, biofilm activity in the distribution system. To investigate the possibility that biofilm was a significant cause of the coliform and E. coli findings, the City Department of Environmental Protection established a biofilm high velocity flushing project in 1995. This paper presents the results of the two-year project which included field sampling and testing, and laboratory analysis for a broad spectrum of microbial and chemical parameters. The aims of the study were to determine if total coliforms and E. coli could be recovered from the distribution system biofilm using high velocity flushing and to determine the relationship between coliform findings in routine bacterial compliance sampling and high velocity flushing sampling.