Evidence for linkage of asthma and its associated phenotypes with susceptibility genes on chromosome 12 has been demonstrated in one group of Minnesota families. The evidence is strong in affected sib pairs and weakens in analysis of the large pedigrees. A second group of families provided little evidence for such linkage. A discrepancy has been demonstrated in different families. This may be due to several factors, including genetic heterogeneity and gene–gene, or gene–environmental interaction effects as well as the statistical power of the sample population used.