The incidence of a positive family history with asthma and levels of serum IgE and IgE antibodies were examined in 136 patients with asthma in relation to age at onset of the disease. The frequency of subjects with a family history of asthma ranged from 37.9 to 75.0% in all groups classified by age at onset. The frequency of patients with a high serum IgE level (≥150IU/mL) was higher (51.7-63.2%) in all groups than the frequency of patients with a low serum level (<150IU/mL). The mean level of serum IgE was significantly higher in patients with a family history than in those without a family history, in subjects between the ages of 50 and 59 years at onset (mean age 63.4 years; P<0.02) and in those over the age of 60 years at onset (74.0 years; P<0.01). The number of patients with a positive RAST score either to house dust mite (HDM), cockroach, and Candida tended to decrease as the age at onset increased. However, the frequency of positive RAST to HDM was higher in patients with a family history and who were over the age of 50 years at onset compared with those patients between the ages of 40 and 49 years at onset, although the frequency was significantly higher in patients with family history than in those without family history (P<0.02). These results suggest that IgE-mediated allergic reactions are significant not only in those patients who are younger, but also in elderly patients with asthma.