BACKGROUND:: Conceptual apraxia (CA), a feature of Alzheimer disease (AD), can be detected by asking participants to identify the correct tool to act on an object. Assessment can be based on either learned associations (a tool selection test) or the mechanical properties that the tool needs to alter the target object (an alternative tool selection test). OBJECTIVES:: We wanted to determine whether knowledge of semantic taxonomic relations (intrinsic properties shared by items) correlated with performance on tests for CA in people with AD or amnestic mild cognitive impairment (aMCI). METHODS:: We tested 10 participants with AD, 12 with aMCI, and 18 healthy older adults for CA using an alternative tool selection test, a tool selection test, and a test of taxonomic relations. RESULTS:: The aMCI group did not differ from the control group on the CA tests. The patients with AD were impaired on all tests except tool selection; their performance on the alternative tool selection test correlated significantly with their performance on the taxonomic relations test. CONCLUSIONS:: The correlation between performances on the alternative tool selection test and the taxonomic relations test in AD suggests a common pathophysiologic substrate, either impairment in accessing conceptual-semantic representations or a degradation of these representations. Copyright © 2012 by Lippincott Williams & Wilkins.