Samples collected every two weeks from the inlet and outlet of three sewage treatment plants were screened for the presence of noro-, rota-, astro-, adeno-, hepatitis A- and circoviruses by (RT)-nested PCR, and for F-specific bacteriophages by isolation in Escherichia coli Famp. Plants A and B were secondary treatment plants and plant C used primary treatment. Noroviruses were detected in 43%, 53% and 24% of the inlet samples and 26%, 40% and 21% of the outlet samples from plants A, B and C, respectively. Astroviruses, rotaviruses and adenoviruses were more prevalent. Adenoviruses were detected in 96% of inlet and 94% of outlet samples, supporting the potential of these viruses as indicators of viral contamination from sewage. Hepatitis A virus and circoviruses were found only rarely. Reduction of infective viral particles during sewage treatment was evaluated using F-specific bacteriophages. The phages were reduced by, respectively, 99%, 87% and 0% in plants A, B and C, which corresponded to the observed differences in reduction of norovirus positive samples between the same plants. The study shows that the high viral load in sewage results in a discharge to the environment of a large amount of virus despite sewage treatment. On the other hand, the advantage of a more advanced treatment is demonstrated.