A 46,X,idic(X)(p11) karyotype was found in a female affected by Turner syndrome and sporadic moderate hemophilia A. Restriction fragment length polymorphism analysis of the patients's DNA demonstrated that the idic(X) contained alleles from both maternal X chromosomes. Since the idic(X) appeared to be always inactivated, a de novo mutation of factor VIII in the normal paternal X chromosome is probably responsible for the patient's coagulation disorder.