X-linked adrenoleukodystrophy (X-ALD), which was first described in 1923, was viewed until 1976 as a rare and inexorably fatal neurodegenerative disorder that affected boys. The genetic defect and biochemical abnormalities have now been defined. Ongoing research has resulted in new findings: (1) there is a wide range of phenotypic expression. At least half of patients with X-ALD are adults with somewhat milder manifestations, and women who are carriers may become symptomatic. X-ALD is often misdiagnosed as attention-deficit/hyperactivity disorder in boys and as multiple sclerosis in men and women, and is not an uncommon cause of Addison disease; (2) the incidence of X-ALD, estimated to be 1:17 000 in all ethnic groups, approximates that of phenylketonuria; (3) noninvasive and presymptomatic diagnosis and prenatal diagnosis are available; family screening and genetic counseling are key to disease prevention; and (4) new therapies, applied early, show promise. Neonatal screening is likely to become available, and a wider awareness of X-ALD and its various modes of presentation permit new proactive approaches to this distressing disorder.