Alzheimer's disease (AD) patients with semantic memory difficulty and AD patients with relatively preserved semantic memory named pictures and judged the category membership of words and pictures of natural kinds and manufactured artifacts that varied in their representativeness. Only semantically impaired patients were insensitive to representativeness in their category judgments. AD subgroup judgments did not differ for natural kinds compared to manufactured artifacts nor for words compared to pictures. AD subgroup differences could not be explained by dementia severity, memory, reading, and visuoperception. The similarity process for relating coordinate members of a taxonomic category contributes to the normal appreciation of word and picture meaning, and this process is compromised in AD patients with semantic difficulty.