Dopa-responsive dystonia is a hereditary disease characterized by inadequate dopamine production. Autosomal-dominant cases result from mutations in the GCH1 gene, encoding guanosine triphosphate (GTP)-cyclohydrolase 1. The most common presenting manifestation is dystonia of a lower extremity, often worsening late in the day. The onset and clinical severity are variable, sometimes even within a single family. Gender effects on allele penetrance have been reported. We present a male toddler with dopa-responsive dystonia caused by an autosomal-dominant GCH1 mutation. Three other family members were also found to carry the mutation, with widely different functional consequences.