BACKGROUND: Scattered reports suggest isolated difficulty with knowledge of numbers and knowledge of objects. We hypothesize that these domains of knowledge involve different forms of cognitive representation, and therefore should be dissociable in patients with specific neurodegenerative diseases. METHODS: We evaluated performance on several measures requiring number knowledge and on an object naming task in patients with Corticobasal Degeneration (CBD; n=13) and Semantic Dementia (SD; n=15). RESULTS: Single subject and group-wide analyses of numerosity magnitude and simple addition involving Arabic numbers and dot arrays consistently showed greater difficulty in CBD than SD. By comparison, SD patients demonstrated a relative impairment in their performance naming familiar objects compared to CBD patients. CONCLUSION: The double dissociation between CBD and SD suggests that number knowledge and object knowledge constitute distinct semantic categories, consistent with a model of semantic memory involving dissociable neural representations of semantic knowledge.