Forests are known to be critically important functions in terms of ecological, economical and social functions they serve. Forest ecosystems play a crucial role in the production of high quality and quantitiy water. Forests lands contain the necessary mechanisms to provide clean, healthy water, including the filtering effect provided by forests soils in watersheds. The objective of this study was to estimate the water filtration of forest soil in Macka Ormanustu Forest Research area, located in northeastern Turkey, which provides public drinking water supply to Trabzon province. For this purpose, the amount of filtered water which contributes to the stream flow per unit area was estimated by using water balance equation including annual precipitation, interception, surface runoff and the amount of water stored in the soil (AWHC). Soil properties such as soil depth, coarse fragment, texture, structure, pH, organic matter and avaliable water hoilding capacity (AWHC) were also determined in forest and open (grassland) area. Results from this study showed that filtered water was approximately 3751.2 m(3) ha(-1) yr(-1) in forest and 2986.1 m(3) ha(-1) yr(-1) in open area. Our results revealed that forests can reduce drinking water treatment costs by naturally filtering water through its porous soils.