Severe factor IX deficiency (Christmas disease) in a phenotypically normal girl prompted a cytogenetic study of the proband and her parents which showed, in the patient, a karyotype with 46 chromosomes (XX ring) and a karyotype comprised of 45 chromosomes with a single X chromosome (45XO). The parents were cytogenetically normal. The activity of factor IX in the patient was markedly abnormal (less than 1%). It is suggested that XX (ring)XO mosaicism permitted expression of the abnormal X-linked gene.