There has been gradual increase of interest in olfactory dysfunction since it was realised that anosmia was a common feature of idiopathic Parkinson's disease (IPD) and Alzheimer-type dementia. It is an intriguing observation that a premonitory sign of a disorder hitherto regarded as one of movement or cognition may be that of disturbed sense of smell. In this review of aging, IPD, parkinsonian syndromes, tremor, Alzheimer's disease (AD), motor neuron disease (MND), Huntington's chorea (HC) and inherited ataxia, the following observations are made: (1) olfactory senescence starts at about the age of 36 years in both sexes and accelerates with advancing years, involving pleasant odours preferentially; (2) olfactory dysfunction is near-universal, early and often severe in IPD and AD developing before any movement or cognitive disorder; (3) normal smell identification in IPD is rare and should prompt review of diagnosis unless the patient is female with tremor-dominant disease; (4) anosmia in suspected progressive supranuclear palsy and corticobasal degeneration is atypical and should likewise provoke diagnostic review; (5) subjects with hyposmia and one ApoE4 allele have an approximate 5-fold increased risk of later AD; (6) impaired sense of smell may be seen in some patients at 50% risk of parkinsonism, and possibly in patients with unexplained hyposmia; (7) smell testing in HC and MND where abnormality may be found is not likely to be of clinical value, and (8) biopsy of olfactory nasal neurons reveals non-specific changes in IPD and AD and at present will not aid diagnosis.