People with the agrammatic variant of Primary Progressive Aphasia (PPA-G) and people with stroke-induced agrammatic aphasia (StrAph) both present with non-fluent speech with grammatical deficits in the presence of spared semantic processing. However, in PPA-G deficits gradually emerge over time due to neurodegenerative disease, while in StrAph deficits occur suddenly due to cerebrovascular lesion. In the present study, we compared online sentence processing in a group of healthy adults (n=16), PPA-G (n=10) and StrAph (n=7). Participants completed a sentence acceptability judgment task while EEG was recorded from 32 scalp electrodes. Half of the sentences contained a morpho-syntactic (n=50) or semantic (n=50) violation. As expected, morpho-syntactic violations elicited a significant, posteriorly centered P600 in the group of healthy adults, indicating intact processes of re-analysis/repair following subject-verb agreement violations. Compared to the healthy controls, the StrAph group showed a delayed P600 with an anterior shift, which suggests successful detection but delayed re-analysis processes. Although the relationship between ERP scalp distribution and underlying neural sources is extremely complex, the anterior shift may reflect increased reliance on domain-general resources supporting re-analysis processes. The PPA-G group showed no significant response to morpho-syntactic violations, suggesting a failure to detect such violations. Comparing results from the two patient groups suggests recruitment of domain-general cognitive resources may be hindered in people with PPA-G due to the more widespread cognitive decline in this group. Semantic violations elicited a significant, centro-parietally distributed N400 in all three participant groups, suggesting semantic processing is preserved in both patient groups.