In this study, we wanted to test the hypothesis that hyposensitization (HS) abrogates the late asthmatic reaction (LAR). We therefore selected 15 asthmatic children (subjects) sensitized to house dust mite (HDM), as proven by positive prick tests and/or specific IgE via the RAST. All children demonstrated a positive bronchial provocation test (BPT) to HDM; 14 showed a dual asthmatic reaction and one child showed an isolated LAR. All children were treated with anti-asthmatic drugs and received HS. They were rechallenged after 1 yr of HS while their anti-asthmatic medication was not changed, except for beta-agonists which were used only for relief of symptoms. Two BPTs, with a mean interval of 6.4 months, were also performed in eight asthmatic children (controls), who did not receive HS. In the controls, the same reaction pattern was observed during both BPTs. In the subjects, the LAR completely resolved in 5/15 after 1 yr of HS (P = 0.04). Furthermore, as a group, the subjects showed a less severe LAR after 1 yr of HS (expressed as mean fall of FEV1) (P less than 0.0001). The PD20 of the immediate asthmatic reaction (IAR) was the same as before HS was started, but the IAR was also less severe after 1 yr of HS (expressed as mean fall of FEV1) (P = 0.028). From these observations it is concluded that, in asthmatic children, HS may resolve and/or reduce the severity of the LAR. Although the PD20 of the IAR is not changed, the severity of the IAR is also reduced after 1 yr of HS.