Age of onset (AO) of Huntington disease (HD) is known to be correlated with the length of an expanded CAG repeat in the HD gene. Apolipoprotein E (APOE) genotype, in turn, is known to influence AO in Alzheimer disease, rendering the APOE gene a likely candidate to affect AO in other neurological diseases too. We therefore determined APOE genotype and normal CAG repeat length in the HD gene for 138 HD patients who were previously analysed with respect to CAG repeat length. Genotyping for APOE was performed blind to clinical information. In addition to highlighting the effect of the normal repeat length upon AO in maternally inherited HD and in male patients, we show that the APOE epsilon2epsilon3 genotype is associated with significantly earlier AO in males than in females. Such a sex difference in AO was not apparent for any of the other APOE genotypes. Our findings suggest that subtle differences in the course of the neurodegeneration in HD may allow interacting genes to exert gender specific effects upon AO.