Forty-two patients with senile dementia of the Alzheimer type (SDAT), 38 patients with Huntington's disease (HD) and matched normal controls were administered tests of olfactory, verbal, and visual recognition after being screened for normal olfactory discrimination. Early-affected Huntington's patients (EHD) with minimal chorea or cognitive deficit displayed marked deficits in olfactory recognition despite normal verbal and visual performance, even after correction for task difficulty, suggesting involvement of olfactory brain regions early in the disease process. In the early Alzheimer's group (EAD), marked deficits were seen on all recognition modalities indicating more global impairment. Both overall (early plus late) patient groups were impaired relative to controls on all recognition tasks, with the olfactory paradigm being most affected.