A 12.9 year-old girl, genotypically 46, XY, and considered to have a testicular feminization syndrome, developed signs of virilization and gynaecomastia. Very high androstenedione concentrations (10-fold the mean of the reference interval in boys) in relation to low normal testosterone in peripheral serum indicated a 17-ketosteroid reductase deficiency. In addition to androstenedione, the basal peripheral levels of 17-hydroxyprogesterone and estrone were increased, being 5- and 3-fold the mean of the reference interval, respectively, whereas pregnenolone, progesterone, dehydroepiandrosterone, 5 alpha-dihydrotestosterone and estradiol concentrations were within pubertal stage-appropriate reference intervals. The total spermatic vein serum steroid concentrations were about 5-fold the mean in old men, and androstenedione, estrone and dehydroepiandrosterone were particularly elevated, whereas estradiol was normal and testosterone subnormal by a factor of 1/8. In the testis tissue, the concentration of androstenedione was extremely high, whereas that of testosterone tended to be relatively low. Our patient was obviously producing testicular steroids at her maximal rate, because no response to hCG administration was observed. This state was associated with a high-normal circulating LH concentration. The concentration of testicular LH/hCG receptors was only one-fifth of that seen in old men, which may have resulted from receptor down-regulation associated with a high degree of stimulation.