A study was conducted to evaluate the quality of water supplied by the Water Supply Company of Jakarta. The monitoring of the water quality was done by both the Jakarta Health Service and the Water Supply Company of Jakarta, based on the Regulation No. 01/BIRHUKMAS/I/1975 issued by the Minister of Health. Several parameters of the quality standard were not met. On the average 0-35% of the samples did not meet the standard for fluoride. Other parameters which deviate from the standard were: hardness (43-93%), ammonium (21-49%), nitrite (27-30%) and organic substances (1-6%). The factors mentioned as possible causes were, aging of distribution pipes, lack of awareness in the community to maintain the water distribution system, water source pollution and limited ability of water processing units in the Water Supply Company.