The prevalence of antibodies to orthopoxvirus in 217 sera collected from domestic cats in the western part of Norway was 10.1 per cent as measured by a competitive ELISA. In one of the seropositive cats antibodies were also detected by an immunofluorescence assay. The average age of the cats sampled was 4.9 years, but the average age of the seropositive individuals was 7.3 years, higher than the average age of clinical cowpox virus cases in Britain (4.2 years), and in Germany (3.9 years). Antibodies against feline immunodeficiency virus (FIV) were detected in nine of 30 (30 per cent) of the seropositive cats, and in five of 30 (17 per cent) of the seronegative cats, which suggests that FIV infection may influence the susceptibility of domestic cats to orthopoxvirus, or vice versa. Orthopoxvirus infections, have recently been detected in rodent populations in several areas of Norway, and the infection may therefore be present in cats all over the country; cat owners and animal handlers should be aware of this (re)emerging zoonosis.