Research on children with Joubert syndrome has focused on brain structural abnormalities and associated clinical symptoms. The degree of developmental delay has not been objectively reported. We investigated the neurobehavioral development of children with Joubert syndrome through neurobehavioral assessment in the largest sample to date. Thirty-two parents of children with Joubert syndrome completed the Child Development Inventory and magnetic resonance imaging (MRI) data was gathered on 17 of these children. Results indicate that 94% were severely impaired according to the Child Development Inventory, with age being positively correlated with degree of neurobehavioral impairment. The average developmental age of our sample was 19 months (63% below chronological age). Severity of illness as measured by the General Development scale of the Child Development Inventory and severity of illness as measured by MRI (overall severity rating) did not yield consistent data regarding severity of the midbrain and cerebellar malformations. Similarly, markers of abnormal cerebral development such as cortical atrophy and delayed myelination were independent of severity of illness ratings on the Child Development Inventory. The degree of developmental delay in Joubert syndrome and the severity of gross central nervous system malformations appear independent.