Age-related changes in IgE-mediated allergic reaction were examined in patients with late onset asthma, whose asthma occured over the age at onset of 40. 1. The number of patients with a high serum IgE level over 3001U/mℓ was larger in those between the ages at onset of 40 and 49 and over age at onset 60 than in those between the ages at onset 50 and 59. 2. A positive RAST score to Hdm and cockroach allergens was more frequently found in patients between 40 and 49 and over age 60 at onset compared to those between 50 and 59 at onset. In contrast, the number of patients with a positive RAST to Candida was larger in those between 50 and 59 at onset than in those of between 40 and 49 and over 60 at onset. 3. Severe asthma was more frequently and slight asthma was less frequently observed in patients between 50 and 59 at onset than in those of the other age at onset groups. These results suggest that IgE-mediated allergic reaction in patients between the ages at onset of 50 and 59 is different from that in those with early onset asthma, and that the reaction in patients between the ages at onset 40 and 49 and over 60 resembles to the reaction in those with early onset asthma.