An 18 year old phenotypic man is described with chromatin-positive Klinefelter's syndrome and undetectable peripheral human follicle stimulating hormone levels. The subject manifested chromosomal mosalcism consisting of three stem cell lines (45X; 46XY; and 47XXY). Testicular biopsy specimen showed germinal cell aplasia: the tubules were lined by Sertoli cells only, whereas the Leydig cells appeared normal. Serum human follicle stimulating hormone levels were undetectable and rose to only 5 mlU/ml after the administration of luteinizing hormone releasing hormone. Serum human luteinizing hormone varied between normal and moderately elevated values, and serum testosterone was in the low normal range. We discuss the features which distinguish this syndrome from isolated gonadotropin deficiency and from classic germinal cell aplasia. We suggest that the patient represents a new variant of Klinefelter's syndrome, with failure of human follicle stimulating hormone release secondary to prolonged hypersecretion.