We have replicated the observation of McMahon et al. [1995] that there is excess maternal transmission of illness in a series of previously described unilineal Bipolar manic-depressive illness extended pedigrees [Berrettini et al., 1991]. ("Transmission" is defined for any ill person in a pedigree when father or mother has a personal or immediate family history of major affective disorder.) We divided our pedigrees into exclusively maternal transmission (Mat) and mixed maternal-paternal transmission (in different pedigree branches) (Pat). Using affected sib-pair-analysis, linkage to a series of markers on chromosome 18p-cen was observed in the Pat but not the Mat pedigrees, with significantly greater identity by descent (IBD) at these markers in the Pat pedigrees. As compared with the pedigree series as a whole, the proportion of alleles IBD in the linkage region is much increased in the Pat pedigrees. As shown by Kruglyak and Lander [1995], as the sharing proportion of alleles in affected relative pairs increases, the number of such pairs needed to resolve the linkage region to a 1 cM interval becomes smaller. Genetic subdivision of an illness by clinical or pedigree configuration criteria may thus play an important role in discovery of disease susceptibility mutations.