Double aneuploidy involving Down and Turner syndromes is a rare occurrence. Of the six patients reported to have combined Down and Turner syndromes, four fundamentally different forms of chromosome mosaicism have been noted and all have been mosaic with respect to monosomy X. Reported here is the first example of a Turner-Down patient in whom there is no X mosaicism. The different forms of the double aneuploidy cannot be explained by any single combination of nondisjunctional errors. The clinical findings in these patients and the several mechanisms of nondisjunctional error that may account for the observed forms of aneuploidy are reviewed and discussed.