A recently described, distinct form of male sexual precocity is characterized by premature Leydig and germinal cell maturation in the absence of pituitary gonadotropin stimulation. Analysis of a nine-generation kindred with at least 28 affected males supports sex-limited autosomal dominant transmission. Three boys, with precocious sexual development at 1 to 4 years of age, had low basal plasma gonadotropin values without pubertal-type pulsatility and a minimal rise in luteinizing hormone after acute stimulation with luteinizing hormone releasing factor or its potent analog D-Trp6-Pro9-NEt-LRF, distinguishing them from boys with true precocious puberty. Two affected adults had a mature luteinizing hormone response to LRF. Testicular biopsies showed a progression of abnormalities in the seminiferous tubules from childhood to maturity; in one adult this disorder was associated with marked oligospermia and selective elevation of plasma follicle-stimulating hormone. The findings are consistent with an inherited intratesticular defect. Furthermore, the majority of cases of familial male sexual precocity seem to be examples of this disorder rather than central or true precocious puberty.