In 2 patients with congenital testicular hypoplasia, the nuclei were chromatin-positive (female chromatin pattern) in skin biopsies, in oral smears, and in the Leydig cells of testis biopsies. Severe hypoplasia of the seminiferous tubules was present in both subjects. One of them satisfied the requirements for a diagnosis of Klinefelter''s syndrome. These patients were thought to represent errors of sex development, on a genetic basis, in embryos having 2 X-chromosomes.