Juvenile Huntington disease (JHD) is a rare clinical entity characterized by disease onset before the age of 21. JHD accounts for <10% of Huntington disease patients. Transmission of JHD is paternal in 80–90% of cases. Patients with JHD usually carry more than 60 CAG repeats within the HTT gene. We report here on a 23-month-old boy presenting with global developmental delay first noted at 18 months of age. Clinical examination showed truncal hypotonia, postural and intentional tremor, limb rigidity, and ataxia. Cerebral magnetic resonance imagery (MRI) showed reduced cerebellar volume. Six months later, his 47-year-old father was seen for a 4-year history of progressive dementia with severe behavioral disturbance and chorea. Cerebral MRI showed discrete global and caudate atrophy. DNA analysis revealed a very large and heterogeneous expansion (210–250 CAG) in the child and a 43 CAG expansion of the HTT gene in the father. This unusual case demonstrates that very early onset JHD due to large CAG expansions should be considered in cases of global developmental delay associated with reduced cerebellar volume, including cases without known HD family history. © 2011 Wiley-Liss, Inc.