Recently, the testicular regression syndrome has been reported as being representative of a clinical range of XY agonadal persons. A patient with aberrant fetal sexual development is described. Of particular interest is that all of the tissues examined—the rudimentary uterine horn and the 2 fallopian tubes—displayed an XY chromosome complement. Correlation was made between the known temporal events of embryogenesis of the testes and the apparent time of regression of the testes. In this patient, the insult to testicular development, with subsequent testicular regression, occurred between 48 and 60 days after fertilization. This review supports the previously reported thesis that in certain XY persons the testes are irreparably damaged at a critical stage in fetal development, and that this critical stage is represented by a range of abnormalities of genital development. Possible causes and simplified nomenclature for such gonadal damage are presented.