The San Mateo landfill has been discharging leachate into the Bosoboso River since it started operating in 1991. The emuent being discharged may potentially deteriorate the receiving water systems in the area. This study, which was conducted from August to December 1998. was aimed at assessing the impact of the San Mateo landfill on its receiving water systems as well as the receiving waters of the Bosoboso rivers using the following parameters: volume of discharge, conductivity. pH, turbidity, dissolved oxygen (DO). Chemical Oxygen Demand (COD). iron, copper and zinc in the landfill leachate. The results of the study revealed the following. High conductivity readings were obtained in the landfill (highest value of 8.32 mS/cm) in comparison with other sites (0.291 mS/cm) indicating a high ion content in the leachate. The pH of all samples taken from the sites (pH range = 7.69 - 8.39l met the standard pH range of 6.00 - 9.00 for emuent waters as specified in the DENR Administrative Order 35. No significant differences in turbidity were observed among the sampling sites throughout the study. However, DO measurements showed very low values for the landfill site in comparison to the other sites, which is indicative of the presence of very high levels of organic matter. Very high values were observed for COD (3,910 ppm) in the landfill leachate. On the other hand, copper (0.252 ppm) and zinc (1.147 ppm) were found to be within the emuent limits. The high COD level was found to be one major cause for the deteriorating water quality of the Bosoboso River. In the light of proposals to tap this river system as a source of drinking water for Metro Manila. these results highlight the need to implement a Water Quality Management Program to prevent further degradation of the river,