The history, physical appearance and laboratory studies performed on eight boys with premature masculinization are presented. The histologic appearance of their testicular tissue is described. Four of the patients had true precocious puberty, which was complete in one and incomplete in three of them. Four had pseudoprecocious puberty, due to adrenal hyperplasia in three of the patients; two of these had associated metabolic disturbances; in the fourth child pseudoprecocity resulted from an adrenal cortical carcinoma. Comparison of the two groups of patients shows that physically they are similar, although bilateral enlargement of the testes or metabolic disturbances in some instances may indicate the source of the precocity. The children with adrenal virilism excreted large amounts of 17-ketosteroids, possibly arising from abnormal precursors, while the patients with true precocity excreted only small amounts of 17-ketosteroids. The observations further suggest that testicular biopsy may be a simple additional means of distinguishing between true precocity and adrenal precocity. In the former the stimulation seems to be all inclusive with respect to the gonads, leading to the development of large interstitial cells and to tubular maturation and spermatogenic activity of varying degree. In precocity of adrenal origin the interstitial cells do not develop. The appearance of the interstitial cells is apparently the essential feature which differentiates the histology of the testis in these two conditions.