The task of deciphering the genetics of asthma is very complex. Recent studies of the familiar segregation of asthma showed that no single gene accounts for a major part of the expression of the disease, and that a polygenic model with some evidence of an oligogenic influence (i.e., a handful of loci being responsible for most of the genetic control) provided the best fit to the data. Although a final common pathway of recurrent bronchial obstruction is present in most cases of asthma, the disease shows marked phenotypic variability, suggesting etiologic heterogeneity and strong environmental influences. In an effort to circumvent these obstacles, linkage studies for genes controlling for apparently simpler phenotypes have been attempted. Total serum immunoglobulin E (IgE) levels, for example, show strong familiar aggregation and are known to be strongly correlated with asthma risk. Recent epidemiologic studies have suggested, however, that the inherited component of total serum IgE may be of little relevance as a determinant of asthma. Sensitization to certain aeroallergens is also associated with increased prevalence of asthma and is likely to have a genetic component, but the aeroallergens involved vary markedly with locale. In addition, sensitization to aeroallergens occurring at an early age is more strongly associated with asthma risk than late allergic sensitization, suggesting genetic heterogeneity. Therefore, studies of the genetics of phenotypes known to be strongly associated with asthma may clarify the causal role (if any) of the genes regulating their expression in the pathogenesis of asthma.