The familial spastic paraplegias (FSPs) are heterogeneous neurologic disorders that are known to occur clinically as "pure" or "complicated" forms. Although some of the complicated FSPs have been linked to specific metabolic defects, the pure forms of this disorder remain idiopathic and are considered to be primary system degenerations. We report the case of a 28-year-old man who has evidenced a neurologically pure spastic paraparesis since age twenty-five. Consistent with this diagnosis were the findings of magnetic resonance imaging, which revealed atrophy of the thoracic spinal cord without evidence for white matter disease in the cerebrum, cerebellum, or brainstem. His 32-year-old brother has also evidenced progressive spastic paraparesis since age 30, but his case is confounded by a severe head injury at age 24 that caused a skull fracture and a focal demyelinating lesion of the right frontal lobe. Both patients have had hypogonadism, requiring treatment with testosterone, since age 20. Measurement of plasma levels of very long-chain fatty acids (VLCFA) revealed that both brothers had concentrations diagnostic of adrenoleukodystrophy; their mother had plasma VLCFA levels in the heterozygous range. We conclude that neurologically pure FSP can be an early manifestation of adrenoleukodystrophy and that levels of plasma VLCFA should be determined for all cases of FSP in which X-linked inheritance appears tenable. These considerations may have bearing on the ongoing linkage studies for these disorders.