Résumé : Heredity is an important factor in vulnerability to manic depression. A genetic linkage has been demonstrated between manic depression and coagulation factor IX at Xq27 with a TaqI polymorphism at the F9 locus in DNA samples from peripheral leucocytes of manic depressive probands and relatives in 10 informative families. Statistical analysis of the pedigrees gave a maximum lod score of 3·10 at a recombination fraction of 0·11, demonstrating a linkage between a manic depressive locus and the F9 locus in the Xq27 region. © 1987.