The Sertoli-cell-only syndrome is characterized histologically by complete loss of the germinal epithelium in testicular tubules, and clinically by aspermia, i.e. lack of spermatozoa and their earlier stages in the ejaculate. In our opinion the term Sertoli-cell-only syndrome should be used exclusively for these lesions, although some authors also use it for cases with oligo- or azoospermia in which the tubules still retain some remnants of germinal epithelium. The etiology of the Sertoli-cell-only syndrome is obscure. Maldevelopment, with failure of the primordial germ cells to migrate into the future gonads, and secondary destruction of the germinal epithelial layer are discussed. In the hope of finding distinct histologic differences that would indicate either malformation or an acquired lesion, we examined 53 cases of Sertoli-cell-only syndrome out of a series of 2700 testicular biopsies from our Institute. All 53 patients had aspermia, and no remnants of germ cells were identifiable in the testicular biopsies. The microscopic lesions in all cases were more or less identical. Two small groups could be distinguished in which the histologic alterations appeared to fulfil the criteria for germ cell aplasia and germ cell atrophy respectively. The majority, however, belonged to an intermediate group showing all states of transition between these lesions. It is likely that the varying histological patterns simply represent different grades of testicular destruction with loss of germinal epithelium, rather than different etiologies. Our study does not allow conclusions as to the cause of the Sertoli-cell-only syndrome.