AN infant with mixed gonadal dysgenesis was found to have a 45,X/46,Xpsu dic(Y) karyotype. A low level (8%) of mosaicism for the dic(Y) cell line was observed in peripheral blood lymphocytes and skin fibroblasts. The dicentric nature of the Y chromosome became apparent in fluorescence in situ hybridization studies. The presence of Y centromeric sequences was demonstrated in the paraffin-embedded testis and streak ovary sections. The ration of Y-positive cells was higher in the testis than in the streak ovary.