The primary concern is still acute gastrointestinal disease associated with the consumption of drinking water in the world. In this study, we evaluated the performance of water treatment works and the drinking water samples in terms of indicator and pathogenic bacteria in Istanbul. Between October 2002 and April 2006, we collected a total of 2237 water samples derived from 134 raw water samples, 446 mains distribution samples from Omerli, Emirli, Elmali, Kagithane, Ikitelli and B.cekmece purification plants and 1657 networks water samples. In these samples were investigated faecal E. coli (Escherichia coli) and coliform bacteria and, additionally, enteropathogenic E. coli (EPEC), Salmonella sp., Shigella sp., Vibrio cholerae, Vibrio sp., Yersinia enterocolytica, Campylobacter sp., Aeromonas sp., Pseudomonas sp. and Plesiomonas sp. The results were evaluated according to the standardization of Turkey (TS 266) (Regulation Concerning Water Intended for Human Consumption). In raw water samples, 100% coliform bacteria and 46% faecal E. coli were found. Additionally, Aeromonas sp., Pseudomonas sp., Vibrio sp., Plesiomonas sp., Yersinia sp. and Shigella sp. were positive in 97%, 67%, 8%, 5%, 1% and 1% of these water samples, respectively. In the sample from main distributions of purification plants and networks water were determined 4% and 5% coliform bacteria, 1% and 2% faecal E. coli, 16% and 23% Aeromonas, 10% and 14% Pseudomonas strains, respectively. EPEC, Salmonella, Shigella, Yersinia, Campylobacter, Vibrio and Plesiomonas sp. were not detected in these samples. In conclusion, all of raw water samples were found to be contaminated in Istanbul. On the contrary, it was observed that 95% of networks water samples are safe according to TS 266. We determined that the water treatment works in purification plants usually effectives in terms of bacteriologic, but it is not sufficiently effective for Aeromonas which might be a potential risk for public health.