The aim of this study was to assess the distribution of the occurrence of tolerance to the protective effect of salmeterol on allergen challenge in a large sample of asthmatic subjects. We investigated 53 subjects (45 male and eight female), mean age 24+/-8.2 years, with mild intermittent asthma, in stable phase of the disease, never previously treated with regular beta2-agonists. All subjects with a previous positive early airway response (EAR) to a screening allergen challenge underwent, in double blind randomized, cross-over manner, three further allergen challenges: after placebo (T0), after a single dose (50 microg) of inhaled salmeterol (T1), and after regular treatment with inhaled salmeterol (50 microg bid) for 1 week (T2). All subjects showed an EAR after placebo treatment (T0), and were completely protected against EAR by the single dose of salmeterol (T1). After 1-week regular treatment with salmeterol (T2). 24 out of 53 subjects (45%) were still protected, whereas 29 subjects (55%) showed a significant EAR. The distribution of the response to allergen challenge, which was quite homogeneous at T0 and T1, showed considerable heterogeneity at T2. Tolerance to the protective effect of salmeterol on allergen challenge can be observed in a large group of previously untreated mild asthmatic subjects. This phenomenon is heterogeneously distributed, with some subjects still showing a complete protection similar to that obtained after a single dose of salmeterol and others showing a response similar to that obtained after placebo. The reason of this heterogeneity needs to be elucidated.