Twelve anorchid pre-pubertal and post-pubertal patients, aged between 8.5 and 41 years, were studied. The FSH and LH basal levels as well as their responses to Gn-RH (100 micrograms i.v.) were increased and prolonged in the post-pubertal patients. The testosterone levels were in the pre-pubertal range and failed to increase after HCG administration (1500 I.U. t.i.d. for 3 days). Ultrasonography of the pelvis was performed in all the patients. Computed tomography and magnetic resonance of the abdomen were also performed in 6 and 5 patients respectively. These techniques failed to show any testicular tissue. Lastly, in 4 patients, surgical exploration confirmed the absence of testicular structure.