House dust mite-specific IgE, IgG and IgG subclass antibody responses were evaluated during immunotherapy in perennial allergic rhinitis patients. It was found that IgG4 antibodies steadily increased during treatment, while IgG and IgG1 antibodies reached a plateau 6 months after the initiation of immunotherapy. IgE antibodies also increased during treatment. It was also found that the good clinical outcome was associated with the rate of increase of IgG4 antibodies, but not with IgG, IgG1 or IgE antibodies.