A positive association has recently been reported in adult subjects between O/nonSecretor phenotype and asthma. To confirm this association, this study investigated the joint ABO/Secretor phenotype in a cohort of 165 asthmatic children. Three-hundred and sixty-two consecutive newborn infants from the same population were also studied as controls. The proportion of O/nonSecretor in asthmatic children was higher than in controls, thus confirming the association found in adults. The association was more marked in males than in females. In males, the pattern of association between the joint ABO/Secretor phenotype and asthma is dependent on the age at on-set of symptoms. Since the oligosaccharide composition of cell membrane and mucosal secretions is controlled by the cooperative interaction of ABO and Secretor genes, and since such composition influences the adhesion of infectious agents, the age pattern could reflect a more general interaction between developmental maturation and oligosaccharide structure concerning their effects on susceptibility to viral and bacterial agents.