We report a 4-year-old girl heterozygous for X-linked adrenoleukodystrophy (ALD) who displayed dopa-responsive motor symptoms and was subsequently diagnosed with sepiapterin reductase deficiency (SPR; OMIM 182125). Her father and paternal uncle had known ALD, and she was found to have elevated plasma very long chain fatty acids and a mutation in the ABCD1 gene. She had language delay, severe hypotonia and abnormal eye movements and responded dramatically to a trial of levodopa (4 mg/kg per day). Two mutations within the gene for sepiapterin reductase were found and cultured skin fibroblasts demonstrated near zero activity of the enzyme. This case illustrates the importance of treatment trials that may give rise to biochemical clues to the underlying diagnosis, and the need to continue to search for diagnoses despite a misleading family history.