Purpose: This study was conducted to evaluate the clinical utility of error variability for differentiating between apraxia of speech (AOS) and aphasia with phonemic paraphasia. Method: Participants were 32 individuals with aphasia after left cerebral injury. Diagnostic groups were formed on the basis of operationalized measures of recognized articulatory and prosodic characteristics of AOS and phonemic paraphasia. Sequential repetitions of multisyllabic words were elicited as part of a motor speech evaluation and transcribed phonetically. Four metrics of variability at the syllable and word levels were derived from these transcripts. Results: The measures yielded different magnitudes of variability. There were no group differences between participants who displayed speech profiles consistent with AOS and participants who displayed speech profiles indicative of aphasia with phonemic paraphasia. Rather, correlation coefficients and analyses of covariance showed that the variability metrics were significantly mediated by overall error rate. Additionally, variability scores for individuals with salient diagnoses of AOS and conduction aphasia were inconsistent with current diagnostic guidelines. Conclusions: The results do not support diagnostic validity of error variability for differentiating between AOS and aphasia with phonemic paraphasia. Future research using error variability metrics should account for overall error rate in the analysis and matching of participant groups. (Contains 8 tables and 1 figure.)