Whereas adult-onset Huntington disease is a well-characterized clinical entity, childhood-onset cases have not received as much attention. In this report, the clinical, demographic, and genetic characteristics in 12 patients with childhood-onset Huntington disease are presented and compared with data in the literature. The patients were divided into two groups based on age at onset of symptoms (< 10 or > or = 10 years old). The majority of patients had onset of symptoms before 10 years of age and most at or below 5 years of age. The delay in diagnosis was longer in those with earlier onset of symptoms. Inheritance was paternal in all patients with onset beyond 10 years of age. We found a preponderance of male patients in the younger age at onset group and of female patients in the older age at onset group. The most frequent heralding symptom was cognitive decline in the group with earlier onset and oropharyngeal dysfunction in the later-onset group. Seizures occurred only in the younger age at onset group. Chorea was not a presenting sign but developed later in the course of the disease and, with dystonia, was more prevalent in the early age at onset group, whereas rigidity and bradykinesia were more prevalent in the older age at onset group. Patients in both groups developed gait, cognitive, and behavioral disorders at some point during the course of the disease. Furthermore, a slow and steady decline in IQ was observed on serial neuropsychologic testing in patients from both groups. Imaging studies were normal early and most commonly revealed neostriatal atrophy later in the course of the disease. In this report, we describe the characteristics of 12 patients with childhood-onset Huntington disease and review those previously reported, expanding our knowledge about the features of childhood-onset Huntington disease, underlining the differences with patients with adult-onset Huntington disease, and suggesting a differential phenotype within patients with childhood-onset Huntington disease depending on the age at onset.