To examine the roles of genetic polymorphism of the beta2-adrenergic receptor (beta2AR) in the relationship between eosinophil (EOS) counts and eosinophil cationic protein (ECP) counts and lung function, we recruited a random sample from the 1996 nationwide survey of asthma prevalence in middle school children. A total of 149 subjects--42 asthmatic children, 38 asthmatics in remission (no reported attack for more than 12 months), and 69 nonasthmatics--completed a physical evaluation, pulmonary function test, and determination of EOS, ECP, and beta2AR genotypes at amino acids 16 and 27. Asthmatic children had higher EOS and ECP than did nonasthmatics. No association was found between asthma and beta2AR genotypes. Lung function was significantly and inversely correlated with EOS but not with ECP in asthmatic children. By genotype, an inverse correlation between lung function and EOS was found in asthmatic children with Arg16Arg or Gln27Glu. A nonsignificant but similar inverse correlation was found in asthmatic children with Arg16Gly or Gln27Gln. However, a nonsignificant but positive correlation was found in asthmatic children with Gly16Gly. In conclusion, we suggest that EOS is a better clinical indicator of airway inflammation than ECP when children are not having an asthma attack. The association between an increase of EOS and lower lung function can be differentiated by beta2AR genotypes at amino acid 16.