Polymorphism at position 16 of the beta2-adrenoceptor alters receptor down-regulation in vitro. Our aim was to compare the development of tolerance to beta-agonist in homozygous Gly-16 patients with patients harboring the "wild" genotype (homozygous Arg-16) during regular treatment with salmeterol. In a prospective, randomized, double-blind, placebo-controlled, cross-over study, 20 subjects with mild to moderate asthma (10 Gly-16, 10 Arg-16) received 2 weeks of treatment with inhaled salmeterol 100 microg b.i.d. Thereafter, dose responses to inhaled salbutamol were constructed for forced expiratory volume in 1 sec (FEV1), heart rate, QTc interval, serum potassium and glucose, and finger tremor. The protective effect of salbutamol against adenosine monophosphate (AMP) challenge was also measured. Salmeterol resulted in a significant reduction in the area under curve (AUC) for FEV1 (p = 0.01), heart rate (p = 0.01), QTc interval (p = 0.01), and tremor (p = 0.05), and in the maximum responses for FEV1 (p = 0.05), heart rate (p = 0.02), and glucose (p = 0.02). The protective effect of salbutamol against AMP was reduced by 3.61 doubling doses (p < 0.001). However, differences between Gly-16 and Arg-16 patients were small and nonsignificant. Thus, although tolerance is influenced in vitro by polymorphism of the beta2-adrenoceptor, the magnitude of between-genotype differences in vivo is unlikely to be significant.