Performance on tests of odour discrimination, naming, and matching was compared in patients with four distinct forms of neurodegenerative disease: Alzheimer's disease (AD), semantic dementia (SD), frontotemporal dementia (FTD), and corticobasal degeneration (CBD). The SD patients were found to have a severe impairment of identification from olfaction despite having normal discrimination, consistent with the multimodal semantic impairment characteristic of this patient group. The AD patients' poor odour discrimination suggests that a perceptual impairment is the root of their poor odour identification. Mild impairments in odour identification observed in FTD and CBD are consistent with their generalised executive dysfunction. The findings illustrate that breakdown in olfaction can occur at a perceptual or semantic level, analogous to the distinction between apperceptive and associative forms of deficit in the visual and auditory modalities. The findings add further insights into the nature of the semantic deficit in SD by exploring a hitherto neglected modality and may have relevance in explaining the altered eating habits commonly associated with SD.