Down syndrome is the most common chromosomal abnormality in live-born individuals, occurring at a frequency of about 1/800 live births. Here we report on a very rare association in an infant with typical features of Down syndrome and XX testicular disorder of sex development. Chromosomal analysis of the patient showed a 47,XX,+21 karyotype. Fluorescence in-situ hybridization analysis of peripheral blood lymphocytes did not detect chimerism or mosaic cell lines but identified the presence of the SRY gene on the short arm of the X chromosome. Most likely, this combined phenotype had resulted from two separate events: one occurring during maternal meiosis leading to chromosome 21 nondisjunction and the other due to abnormal terminal X–Y interchange during meiosis of the father.