The hypothesis that repeated exposure to a specific allergen will further increase bronchial responsiveness to that allergen is supported by indirect evidence. However, it has not been tested as intensely in the laboratory setting, and in some cases, conflicting results are presented. In order to test the hypothesis in the atopic subjects, allergen inhalation challenge tests were performed in 29 house dust mite (Dermatophagoides pternyssinus) sensitive subjects with allergic rhinitis. Nine subjects displayed early asthmatic responses (EARs) to the first challenge (Group 1). Twenty subjects with no significant airway response were submitted to the second challenge 24 h later. Thirteen subjects showed EARs (Group FI) and two of these showed late asthmatic responses (LARs)aswell. In Group II, there were significant changes between the first and second challenge in post-allergen early phase FEV1, (88.1 ± 4.2 vs 71.7 ± 4.2%, baseline, P < 0.05) and in post-allergen late phase FEV1, (93.1 ± 3.4 vs 86.6 ± 7.8. P < 0.05). After the second challenge. PD20 (provocative dose of methacholine required to produce a 20% fall in FEV1) decreased significantly from the baseline values. When challenged separately with twofold dose of allergen, only three and one of the Group II showed EAR and LAR respectively. PD20 did not change significantly after this challenge. These results indicated that two repeated exposure to allergen dose, which is not enough to cause significant airway responses at a time, may provoke asthmatic airway responses in the subjects with allergic rhinitis and that this effect of priming is not attributed to the cumulative dose but to the consequent effect of repeated allergen exposure.