Three cases of Huntington's chorea with onset before age 10 years are reported. Each child presented with rigidity and indistinct speech, and there was progressive deterioration. Necropsy examination confirmed the diagnosis in 2 of them. A review of reports showed a further 43 cases with onset before 10 years. The rigid variety of disease was seen most often, but isolated chorea and isolated progressive mental deterioration occurred. Fits were common but occurred late and were often difficult to control. Dysarthria was common and occurred early. The duration of illness was very variable and ranged from 2 to 38 years. Symptoms can occur in a child before appearing in the affected parent who is most likely to be the father. Affected siblings develop the disease early, often in the first decade. Siblings of patients with onset before age 10 years who are unaffected by age 25 years had only an 8% chance of developing the disease, compared with a 50% chance in unselected at risk individuals of the same age.