The CAG repeat number on Huntington's disease (HD) chromosomes accounts for about 60% of the variance in the age at onset of HD. However, distinct familial factors may also influence the age at onset. HD is associated with loss of medium-sized GABA-ergic striatal output neurons. Intracerebral administration of human ciliary neurotrophic factor (CNTF) protects striatal output neurons in excitotoxic rodent and primate models of HD induced by intrastriatal quinolinic acid injection. We have examined the effect of a common null mutation in the human CNTF gene on the age of onset of HD using a multiple regression approach that takes into account the CAG repeat number on HD chromosomes. We failed to detect an earlier onset of HD in nine homozygotes and 71 heterozygotes with this CNTF mutation compared with 203 homozygotes with wild-type alleles.