The aim of this study was to investigate memory in patients with mild cognitive impairment (MCI) and mild Alzheimer's disease (AD). Ten patients with MCI, 11 with AD and a group of age and education matched healthy control participants were assessed on a comprehensive battery of semantic memory tests, including traditional semantic memory measures and a non-verbal test of knowledge of object use. The MCI group was impaired on tests of category fluency and all three conditions of an object knowledge test (matching to recipient, function and action), plus a difficult object-naming test. The mild AD group showed additional impairments on traditional measures of semantic memory, including naming high frequency items, comprehension and semantic association. Together these findings suggest that semantic memory impairments occur early in the course of AD, more specifically in patients with “amnesic” MCI, and provide further evidence that impaired category fluency reflects semantic breakdown.