Transmission studies have supported the presence of a susceptibility gene for bipolar (BP) illness on the X-chromosome. Initial linkage studies with color blindness (CB), glucose-6-phosphate dehydrogenase (G6PD) deficiency, and the blood coagulation factor IX (F9) have suggested that a gene for BP illness is located in the Xq27-q28 region. We tested linkage with several DNA markers located in Xq27-q28 in 2 families, MAD3 and MAD4, that previously were linked to F9 and 7 newly ascertained families of BP probands. Linkage was also examined with the gene encoding the alpha 3 subunit of the gamma-amino butyric acid receptor (GABRA3), a candidate gene for BP illness located in this region. The genetic data were analyzed with the LOD score method using age-dependent penetrance of an autosomal dominant disease gene and narrow and broad clinical models. In MAD3 and MAD4 the multipoint LOD score data suggested a localization of a BPI gene again near F9. In the 7 new families the overall linkage data excluded the Xq27-q28 region. However, if the families were grouped according to their proband's phenotype BPI or BPII, a susceptibility gene for BPI disorder at the DXS52-F8 cluster could not be excluded.