We report a female infant born to a mother with incontinentia pigmenti (IP) and a father with haemophilia A, who manifests both disorders. Analysis of peripheral blood DNA from the infant, her mother, and two female relatives with IP showed a highly skewed pattern of X inactivation. Random patterns were observed in the infant's two sisters, who do not have IP and have normal carrier activity of factor VIII. Preferential inactivation of the X chromosome bearing the IP mutation, probably by negative selection, appears to have unmasked the factor VIII mutation on the infant's other X chromosome. This illustrates an unusual mechanism for the manifestation of an X linked disease in a heterozygous female.