Combined sewer overflow (CSO) pollution was assessed in a medium size community (the Town of Malacky) in western Slovakia by means of field monitoring and computer modelling. CSO samples were analyzed for suspended solids, total chemical oxygen demand, 5-day biochemical oxygen demand, total organic carbon, heavy metals (Cu, Zn, Ni, Cd, Pb), and biological and microbiological constituents. Study results identified CSOs as a major source of biodegradable organic pollution impacting on dissolved oxygen in the receiving streams. Cumulative effects of CSO discharges were also observed in the receiving streams, in the form of high organic content of benthic sediments, which indicated the deposition of wastewater-derived sediments flushed out from the sewer system during storm events. These findings have also been confirmed by analyses of biological constituents in the sediment samples collected from the receiving waters. A comparative analysis of various pollution sources indicated that pollution problems in the receiving waters were caused not only by the municipal effluents from the Town of Malacky, but also by diffuse agricultural pollution and release of heavy metals, especially Zn, from the native soils in the surrounding area.