A patient with 48XXXX/49XXXXX mosaicism is presented. Clinical findings include severe growth and developmental retardation, hypertelorism, mongoloid slant of the palpebral fissures, clinodactyly of the fifth fingers, retarded bone age and radio-ulnar synostosis. The findings are similar to those of the cases with a penta-X chromosome complement already described, and are also similar to the signs of the more common 49XXXXY syndrome of males. In both instances the dysmorphic features are less impressive than the mental retardation and the skeletal malformations. This report contributes to a better delineation of the 49XXXXX syndrome. The possible mechanisms of the chromosomal aberration are discussed.