X-linked adrenoleukodystrophy (ALD) is a peroxisomal genetic disorder that causes adrenal insufficiency, demyelination in the central nervous system, and increased levels of very long chain fatty acids in tissues and body fluids. Although most cases appear in childhood as a devastating degenerative disorder or in adulthood as a milder disorder affecting the spinal cord, many patients have adrenal insufficiency prior to the onset of their neurologic deterioration for many years. Addison's disease without neurologic involvement may also remain the only clinical manifestation of X-linked ALD. Because of the prognostic implications, the need for genetic counseling, and the potential benefit of therapeutic interventions, any boy or adult male with Addison's disease must be tested for X-linked ALD.