The antibody prevalence in Sweden to Norwalk virus (NV) was investigated using a baculovirus expressed capsid antigen. One hundred thirty-two serum samples were examined for IgA, total IgG and IgG subclass antibodies to Norwalk virus. In young children, NV IgG antibody prevalence was higher than the IgA prevalence, whereas no difference was found in individuals older than 21 years. The IgG antibody prevalence was 50% in children below 5 years of age and increased to > 80% in individuals older than 10 years of age. To examine the IgG antibody response in more detail, IgG subclass patterns were characterized. IgG 1 predominated in all age groups. IgG 4, usually detected after repeated exposure to antigen, was the second most prevalent subclass, but was only found in individuals older than 21 years of age. IgG 3 subclass antibodies were found in 13% and IgG 2 in 3% of the sera examined. IgG 3 subclass antibodies have been recognized as a marker for recent or ongoing viral infections.