We describe a case of XY sex reversal, gonadal dysgenesis, and gonadoblastoma in a patient with a deletion of 9p24 due to a familial translocation. The rearranged chromosome 9 was inherited from the father; the patient's karyotype was 46,XY,der(9)t(8;9) (p21;p24)pat. A review shows that 6 additional patients with 46,XY sex reversal associated with monosomy of the distal short arm of chromosome 9 have been observed. The observation that all 7 patients with sex reversal share a deletion of the distal short arm of chromosome 9 is consistent with the hypothesis that the region 9p24 contains a gene or genes necessary for male sex determination. This present case narrows the chromosome interval containing a critical sex determination gene to the relatively small region 9p24. A molecular analysis of this region will provide a means to identify a gene involved in male sex determination.