An X-chromosomal predisposing locus to manic-depressive illness has been suggested since 1969 on the basis of the cosegregation of this trait in some families with phenotypic markers, such as color blindness, the glucose-6-phosphate dehydrogenase deficiency, and the coagulation factor IX deficiency. However, the conclusive evidence and the exact location of the putative X-chromosomal locus have remained controversial. We report here a linkage between DNA markers near the coagulation factor IX gene and bipolar disorder in an extended pedigree rising from the genetically isolated population of Finland. A distinct chromosomal hapiotype covering a 20-cM region on Xq24-q27.1 could be demonstrated to segregate with bipolar disorder. These findings should encourage research groups to study extended family materials with Xq24-q27.1 markers to finally resolve the question of the X-chromosomal linkage of bipolar disorder.Bipolar disorder (BP) belongs to a group of affective disorders, which are psychiatric illnesses causing abnormal mood. Bipolar disorder is characterized by episodes of mania (BPI) or hypomania (BPII) and depression. During the episodes, patients experience changes in sleep, appetite, and sexual behavior and abnormalities of endocrine and chronobiological systems. BP aggregates in families, and, in addition to BP, these families frequently show cases of unipolar disorder and schizoaffective disorder (Weissman et al. 1984;Winokur et al. 1995). These three seem to represent different disorders, but they probably share some etiological factors. The lifetime prevalence of BP is 0.6%-1.2% and it is not influenced by sex or race (for review, see Goodwin and Jarnison 1990). The onset of the disorder typically occurs at young adulthood and results in impaired interpersonal, social, and occupational functioning. As many as one-third of the patients have chronic symptoms and evidence of social decline (for review, see Goodwin and Jamison 1990).The etiology of BP is unknown, but twin studies have evidenced the significance of ge3Corresponding author. E-MAIL petra.pekkarinen@ktl.fi; FAX 358 0 4744 480.