Qualitative assessment of bacteriological quality and chlorination status of drinking water in Lahore. Descriptive study. Lahore city from October 2000 to September 2001. A total of 2160 water samples from distribution system were tested from nine different localities of Lahore. These localities represented areas with different socioeconomic conditions (SEC). Twenty water samples were tested from each locality from the same taps each month. All the water samples were subjected to H2S strip test for determination of bacteriological contamination as well as orthotolidine test for detection of chlorine. Four hundred and forty-six (20.64%) samples were positive for bacterial contamination. It was observed that contamination was the maximum in low SEC areas (32.22%), followed by intermediate SEC areas (18.47%) and high SEC areas (11.25%). The difference was found to be statistically significant (p<0.01) among different areas. Only 27.73% samples were chlorinated. Positivity of samples for chlorine was the lowest in areas with low SEC(20.69%) and highest (32.77%) in areas with high SEC, the difference being statistically significant. Maximum contamination was present in samples tested during summer months(June-August) of the year (31.11%), followed by autumn months (September-November) of the year (20.9%), spring months (March-May) of the year (18.7%) and winter months (December-February) of the year (11.85%). Samples tested during summer months showed the highest positivity (77.96%) for chlorine (p<0.001) as compared with other months of year. Among chlorinated samples 12.32% showed bacterial contamination. However, contamination was significantly higher (p<0.001) among non-chlorinated samples. Bacterial contamination of piped water is a significant problem in Lahore. Regular chlorination and monitoring of water supplies can improve it.