Serum levels of IgG subclass and house dust mite (Dermatophagoides pteronyssinus, Dpt) specific IgG4 were evaluated during immunotherapy in asthmatic children. Asthmatic children undergoing long-term immunotherapy (more than 2 years) posed a mean value of total serum IgG4 or Dpt-specific IgG4 antibodies significantly higher than that of patients prior to receiving immunotherapy, asthmatic (placebo) controls, or patients undergoing short-term immunotherapy (less than 1 year) (P less than 0.05). The mean levels of serum Dpt-specific IgG4 in all asthmatic groups were also significantly higher than in the non-allergic controls (P less than 0.01). Moreover, the mean level of Dpt-specific IgG4 tended to increase during immunotherapy. A significant correlation between total serum IgG4 and Dpt-specific IgG4 antibodies was noted (r = 0.6243; P less than 0.001). Serial follow-up reveals that Dpt-specific IgG4 levels usually rose significantly with clinical improvement in asthmatic children during immunotherapy. These results suggest that the anti-mite-specific IgG4 antibody may serve as an indicator for clinical outcome of mite allergy during immunotherapy.