Testicular tissue from 7 juvenile and 6 adult patients with adreno-leukodystrophy (ALD) or its adrenomyeloneuropathic (AMN) variant was examined by light and electron microscopy. The seminiferous tubules of the men exhibited hypocellularity, maturation arrest, or Sertoli cells and spermatogonia only. Ultrastructural examination of these specimens revealed vacuolation of Sertoli cell endoplasmic reticulum. Germ cells also demonstrated vacuolation and necrosis, accompanied by slight tubular atrophy and thickening of the tunica propria. Fibrosis or proteinaceous fluid was observed in the interstitium. The tissue of 5 men and 2 boys contained Leydig cells with cytoplasmic striations, which were detectable with the light microscope. A decrease in the number of Leydig cells was noted in some men. All specimens displayed pathognomonic lamellas and lamellar-lipid profiles in mature, immature, or precursor Leydig cells at the ultrastructural level. The Leydig cell demonstrated the primary morphologic defect in the ALD testis; damage to Sertoli cells appeared to be the initial lesion of seminiferous tubules.