Objectives: A cell lineage in a hermaphrodite infant showing a 46,XX/46,XY karyotype was analyzed to determine the genetic contribution from both parents and identify the underlying mechanism. Study design: Genotype analysis was performed by using 57 microsatellite markers, 39 distributed on 9 autosomes and 18 on the X-chromosome. Two X- and Y-specific markers were analyzed to confirm the presence and percentages of cells containing a Y-chromosome. Results: The proband revealed a single maternal and paternal allele for all of the informative autosomal and X-chromosome markers. Analysis of the X- and Y-specific markers confirmed that 20% of the patient's peripheral blood cells contained the Y chromosome. Conclusions: The data suggest that the patient's hermaphroditism was the result of a mosaic embryo and not a chimera. The most likely mechanism involved 2 separate nondisjunction events, resulting in the loss of 47,XXY cells during early embryonic development. (J Pediatr 2002;140:732-5)