Bilingual and multilingual speakers may alternate between languages within a conversation and within a sentence. This is called code-switching. Fluent adult bilinguals who have acquired both languages in early childhood, code-switch in accordance with conversational discourse/pragmatic principles and with grammatical principles, like those that apply in single languages. The question then arises: do bilingual aphasic speakers demonstrate ‘pathological’ code-switching, that is, code-switching that fails to comply with these conversational and grammatical principles? Previous research has shown that some aphasic bilinguals manifest conversationally impaired code-switching. Few studies have directly addressed grammatical impairment of code-switching, and these studies have inadvertently applied invalid measures of what constitutes grammatical code-switching in healthy proficient bilingual speakers. Here we report the results of a study investigating the grammatical and conversational patterns of code-switching for 2 aphasic bilingual speakers, in comparison with language- and age-matched controls. Code-switching was investigated across five measures (conversation, narrative recount, elicited sentence production, lexical selection and grammaticality judgements) for three sentence types (subject pronoun declaratives, negation, wh- questions). Testing of lexical, syntactic and cognitive abilities was completed in each language. The results demonstrated a clear distinction between conversational impairment and grammatical impairment of code-switching. For these bilingual aphasic speakers, grammatical impairment of code-switching was evidenced in the absence of conversational impairment. Clinical and theoretical implications of the results will be discussed.