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Background : A patient with semantic dementia had difficulty comprehending semantically reversible passive sentences in the Test for the Reception of Grammar (TROG). Investigations of patients with semantic dementia have typically focused on the lexical and semantic components of language. Syntactic abilities have received less attention, presumably because these are reported to be preserved until later stages of the disease. Aim : To localise the possible functional sources of deficient comprehension of passives in a Dutch man (EGY) with semantic dementia. Methods & Procedures : Dutch written sentence picture-matching tasks were used to investigate whether EGY assigned the thematic role of agent on the basis of word order, the presence of auxiliary plus inflected verb morphology, the by phrase, the presence of a locative preposition ( against and after ), or the lexical-semantic specifications of the verbs push and chase . Outcomes & Results : EGY was able to understand reversible active and truncated passive sentences. He was impaired when comprehension depended on understanding the agentive role of the by phrase, although his performance improved when a locative preposition was available to highlight the theme of the action. Conclusions : Intact performance on reversible active sentences indicated spared ability to process canonical word order. EGY's ability to understand truncated passives showed comprehension of the auxiliary and inflected verb, perhaps resulting in a conceptual representation of the passive as an adjectival construction (e.g., the cow is pushed = the pushed cow ). EGY's deficit in comprehending full passive sentences is explained as a failure to process the syntactic and lexical information in the agentive by preposition. In semantic dementia, the division between lexicon and grammar may be more apparent than real. [ABSTRACT FROM AUTHOR]