The contribution of combined sewer overflows (CSO) to the viral contamination of receiving waters was determined. Adenovirus concentrations were determined using the Primary Liver Carcinoma (PLC/PRF/5) cell line and confirmed by Polymerase Chain Reaction (PCR). Norovirus concentration was determined using the Most Probable Number (MPN) and Reverse Transcription-Polymerase Chain Reaction (RT-PCR). Seventy-five water samples were collected during dry weather and 50 samples were collected during wet weather. CSO events significantly increased the concentration of culturable viruses, adenoviruses, and noroviruses in the receiving waters (P < 0.01). During dry weather, 56% of samples were positive for total virus cytopathic effects (CPE), adenoviruses were detected in 41% of the positive cell cultures, and noroviruses in 6% of the concentrates by direct RT-PCR. During wet weather, 100% of the samples were positive by CPE, 84% for adenoviruses, and 40% in the concentrates for norovirus. Our results demonstrate that CSOs can contribute significant viral loading to receiving waters.