X-Linked adrenoleukodystrophy (ALD) is a genetic disease associated with demyelination of the central nervous system, adrenal insufficiency, and accumulation of very long chain fatty acids in tissue and body fluids. ALD is due to mutation of a gene located in Xq28 that encodes a peroxisomal transporter protein of unknown function. The most common phenotype of ALD is the cerebral form (45%) that develops in boys between 5-12 yr. Adrenomyeloneuropathy (AMN) involves the spinal cord and peripheral nerves in young adults (35%). Adrenal insufficiency (Addison's disease) is frequently associated with AMN or cerebral ALD and may remain the only clinical expression of ALD (8% of cases). The prevalence of ALD among adults with Addison's disease remains unknown. To evaluate this prevalence, we performed biochemical analysis of very long chain fatty acids in 14 male patients (age ranging from 12-45 yr at diagnosis) previously diagnosed as having primary idiopathic adrenocortical insufficiency. In 5 of 14 patients (35%), elevated plasma concentrations of very long chain fatty acids were detected. None of these patients had adrenocortical antibodies. By electrophysiological tests and magnetic resonance imaging it was determined that two patients had cerebral ALD, one had adrenomyeloneuropathy with cerebral involvement, and two had preclinical AMN. Our data support the hypothesis that ALD is a frequent cause of idiopathic Addison's disease in children and adults.