The objective of this study was to assess if drinking water meeting currently accepted microbiological standards is the source of gastrointestinal illnesses and to attempt to identify the source(s) of these illnesses. A randomized prospective study was conducted over a period of 16 months (September 1993-December 1994) in a middle class suburban community served by a single water filtration plant. A representative sample of 1400 families were selected and randomly allocated in four groups of 350, to the following regimens: (1) tap water; (2) tap water from a continuously purged tap; (3) bottled plant water; (4) purified bottled water (tap water treated by reverse osmosis or spring water). The water treatment plant produced wather that met or exceeded current North American regulations for drinking water quality. The distribution system was found to be in compliance for both coliforms and chlorine. Using the purified water group as the baseline, the excess of gastrointestinal illness associated with tap water was 14% in the tap group and 19% in the tap-valve group. Children 2-5 years old were the most affected with an excess of 17% in the tap group and 40% in the tap-valve group. Mottled plant water was not the source of any increase in the incidence of gastrointestinal illnesses, even if it contained very high levels of heterotrophic bacteria after two weeks. The data collected suggest that 14-40% of the gastrointestinal illnesses are attributable to tap water meeting current standards and that the water distribution system appears to be partly responsible for these illnesses.