Double monosomy mosaicism was observed in a three-year-old girl who had mental and physical retardation. Routine blood-lymphocyte and bone-marrow chromosome studies showed an apparent 45, X pattern, which was confirmed by autoradiographic studies with tritiated thymidine. Routine and fluorescent chromosome analyses from bilateral skin biopsies revealed a 45,XX,21– karyotype, which was consistent with the positive sex-chromatin pattern in these fibroblasts as well as in the buccal smear and hair-root-sheath cells. Although her clinical features bore some resemblance to patients with Turner’s syndrome and to patients with G monosomy, our patient did not closely fit either of these syndromes.