In previous studies it was demonstrated that the frequency and the severity of the late asthmatic reaction (LAR) can be attenuated by immunotherapy (IT). The present study was set up to observe the evolution of the LAR under IT and after having stopped IT. Nineteen children with bronchial asthma and an LAR to house dust mite (Dermatophagoides pteronyssinus) (HDM) were selected for this study. All subjects received IT with HDM extract during 1 year. Thereafter, the children were divided randomly into two groups to receive, double-blind, during a second year, IT with HDM (N = 9) or placebo injections (N = 10). Bronchial challenges were performed after the first and second year. After the first year, a significant decrease in the severity of the LAR was noted in all but one subject (mean decrease of FEV1 before, 40.53% versus after 1 year, 22.95%; p less than 0.0001). After the second year, the severity of the LAR remained the same in the group that received HDM injections during the second year (20.78% versus 23.00%), but in the group that received placebo injections during 1 year, a significant worsening of the LAR was observed after the second year (24.90% versus 31.20%; p = 0.038). From this study it can be concluded that the severity of the LAR decreases after the first year of IT but that the severity of the LAR remains the same after the second year of IT. In children who stop receiving IT after 1 year, we observed a recurrence of the LAR after 1 year to the same level as before IT was started.