The aim of this study is to compare the power of the transmission disequilibrium test (TDT) to that of the identity-by-descent (IBD) distribution test. The relative powers of these tests depend both on the underlying genetic model and on the available family data. Families with two affected sibs are always more informative than those with one affected child and one unaffected child. The IBD test is always more powerful in the first situation and, contrary to the TDT, is independent of the presence of gametic disequilibrium. When there is strong linkage disequilibrium, the TDT can be more powerful than the IBD test. In that case, linkage can be detected by the TDT even in families with only one affected child. ©1995 Wiley-Liss, Inc.