BACKGROUND: Clopidogrel reduces the risk of non-cardioembolic ischemic stroke, but it is unclear whether it affects the severity and outcome of stroke. We aimed at evaluating the effect of prior treatment with clopidogrel on acute non-cardioembolic ischemic stroke severity and in-hospital outcome. METHODS: We prospectively studied 608 consecutive patients (39.5% males, age 79.1 ± 6.6 years) who were admitted with acute ischemic stroke. The severity of stroke was assessed at admission with the National Institutes of Health Stroke Scale (NIHSS). Severe stroke was defined as NIHSS ≥21. The outcome was assessed using the dependency rates that prevailed at the time of discharge (i.e. modified Rankin scale between 2 and 5) and with in-hospital mortality. RESULTS: At admission, 397 patients did not have atrial fibrillation or heart valve disease. Among these 397 patients, 69 were receiving monotherapy with clopidogrel prior to stroke, 69 were receiving monotherapy with aspirin and 236 patients were not on any antiplatelet treatment. The prevalence of severe stroke was lower in patients who were receiving clopidogrel than in patients who were receiving aspirin and patients who were not on antiplatelets (1.4, 13.0 and 11.0%, respectively; p < 0.05). Independent predictors of severe stroke at admission were male gender (relative risk (RR) 0.31, 95% CI 0.12-0.78, p < 0.05) and treatment with clopidogrel prior to stroke compared with no antiplatelet treatment (RR 0.13, 95% CI 0.02-0.97, p < 0.05). Treatment with aspirin prior to stroke did not predict severe stroke compared with no antiplatelet treatment (RR 1.24, 95% CI 0.51-2.98, p = NS). The rate of dependency at discharge did not differ between patients who were receiving clopidogrel, patients who were receiving aspirin and those who were not on antiplatelets (57.9, 47.8 and 59.7%, respectively; p = NS). Independent predictors of dependency at discharge were age (RR 1.12, 95% CI 1.05-1.19, p < 0.001) and NIHSS at admission (RR 1.67, 95% CI 1.46-1.92, p < 0.001). In-hospital mortality rate also did not differ between patients who were receiving clopidogrel, patients who were receiving aspirin and those who were not on antiplatelets (4.3, 4.3 and 5.0%, respectively; p = NS). The only independent predictor of in-hospital mortality was NIHSS at admission (RR 1.22, 95% CI 1.14-1.30, p < 0.001). CONCLUSIONS: Treatment with clopidogrel prior to acute non-cardioembolic ischemic stroke attenuates the severity of stroke at admission but does not appear to affect the functional outcome at discharge or the in-hospital mortality of these patients.