Purpose: The production of formulaic expressions (conversational speech formulas, pause fillers, idioms, and other fixed expressions) is excessive in the left hemisphere and deficient in the right hemisphere and in subcortical stroke. Speakers with Alzheimer's disease (AD), having functional basal ganglia, reveal abnormally high proportions of formulaic language. Persons with Parkinson's disease (PD), having dysfunctional basal ganglia, were predicted to show impoverished formulaic expressions in contrast to speakers with AD. This study compared participants with PD, participants with AD, and healthy control (HC) participants on protocols probing production and comprehension of formulaic expressions. Method: Spontaneous speech samples were recorded from 16 individuals with PD, 12 individuals with AD, and 18 HC speakers. Structured tests were then administered as probes of comprehension. Results: The PD group had lower proportions of formulaic expressions compared with the AD and HC groups. Comprehension testing yielded opposite contrasts: participants with PD showed significantly higher performance compared with participants with AD and did not differ from HC participants. Conclusions: The finding that PD produced lower proportions of formulaic expressions compared with AD and HC supports formulaic expressions. Contrasting results on formal testing of comprehension, whereby participants with AD performed significantly worse than participants with PD and HC participants, indicate differential effects on procedural and declarative knowledge associated with these neurological conditions.