The effect of maternal transmission on age at onset of Huntington's disease (HD) was examined in 100 unrelated pedigrees. The age at which abnormal movement disorder first appeared could be estimated in 238 patients. More than twice as many of the late-onset cases (age 50 or later) inherited the HD gene from an affected mother than from an affected father. Affected offspring of late-onset females also had late-onset disease while those of late-onset males had significantly earlier ages of onset. This pattern of maternal inheritance suggests a model where the late-onset form of HD is related to a maternally transmitted factor such as the mitochondrion and its genome. © 1983.