One commonly observed neuroanatomical abnormality in adults with Williams syndrome is an enlarged cerebellum relative to a small cerebrum. Our study is the first to examine neuroanatomy in young children with Williams syndrome. Clinical brain MRI was examined in nine young children with Williams syndrome (mean age 21 months, range 7 to 43 months) relative to nine age- and sex-matched normally developing control children (mean age 29 months, range 20 to 42 months), and two children with undiagnosed developmental disorders (6 and 41 months). Two neuroradiologists who were blinded to participant classification, hypotheses, and regions of interest for the study, sorted the brain scans into two groups on the basis of six neuroanatomical criteria. The raters placed more of the MR scans from children with Williams syndrome into a separate group when they analyzed features of the cerebellum, but not when they analyzed other brain regions. Based on their written comments, the raters focused on the large size of the cerebellum in the children with Williams syndrome. The results lead us to suggest that abnormal cerebellar enlargement is evident in those with Williams syndrome at an early age. Our results are discussed relative to the cognitive delays observed in Williams syndrome versus other disorders such as autism, leading us to suggest that the cerebellum may play a role in cognition.