Serum IgE levels and IgE antibody values in relation to patient age were examined in 50 patients with house dust mite (HDm) sensitive asthma, 1. IgE antibody values to HDm were highest in patients between the ages of 10 and 19, and the values generally low in those over the age of 40. 2. High IgE antibody values to HDm were accompanied by high serum IgE levels in patients between 10 and 19. IgE antibody values and serum IgE levels were in general low in patients over age 40. In patient under age 9, serum IgE levels were less than 500 IU/ml in 5 of the 10 subjects despite an increased value of IgE antibodies to HDm. These results suggest that an increased production of IgE antibodies to HDm is usually accompanied by an increased production of total IgE in younger subjects. However, an increase in production of IgE antibodies is not always related to an increase in production of total IgE in patients under age 9. The results also show that production of IgE antibodies to HDm and total IgE is in general low in older patients despite the hyperreactivity to HDm.