A sex chromatin positive woman has stunted growth, primary amenor-rhoea, cubitus valgus, Madelung’s deformity and mental deficiency. A modal chromosome number of 46 was found in cultures from peripheral blood, skin and Fascia lata, but an abnormal very large metacentric chromosome and only 15 chromosomes in the X-6–12 group were observed in karyotypes from cells of all cultures. Large and characteristic bipartite bodies were often present in buccal and vaginal smears, and the size of the polymorphonuclear drumsticks was increased. Autoradiographically the abnormal chromosome was the only “hot” chromosome. The patient was Xga negative although her father was Xga positive. We conclude that the unpaired large chromosome in our patient is an abnormal X chromosome with a partial deletion and a translocation of a piece of an autosome or sex chromosome onto its sh