BACKGROUND: Stroke due to internal carotid artery dissection is considered to have a good prognosis. OBJECTIVE: To determine whether the prognosis of internal carotid artery dissection is worse than classically reported by comparing the characteristics of patients who had an acute ischemic stroke admitted to a population-based primary care center with internal carotid artery occlusion due to either dissection (DO) or atherothrombosis (AO). PATIENTS AND METHODS: Among 3502 patients admitted to our population-based primary care center, DO (n = 73) was diagnosed by angiography or magnetic resonance imaging, while AO (n = 81) was diagnosed by angiography. The characteristics of patients with DO or AO were compared using univariate and multivariate analysis. RESULTS: Patients with DO were younger (mean [SD] age, 44.6 [10] vs. 60.1 [10] years, P<.001), had fewer vascular risk factors, and presented more frequently with global middle cerebral artery territory involvement (42% vs. 17%, P<.05) and less frequently with watershed infarcts (3% vs. 19%, P<.05) than patients with AO. Unexpectedly, patients with DO were noted to have more severe clinical impairment, with an increased frequency of decreased consciousness, and a poorer outcome at 1 month. Multivariate analysis showed that the independent factors associated with DO were age younger than 55 years, nonsmoker, no history of hypertension, headache at presentation, and global aphasia. CONCLUSIONS: Patients with DO are younger and are initially seen with fewer risk factors than patients with AO, but their clinical features and prognosis are worse. Large infarcts involving the whole middle cerebral artery territory that may be due to the lack of collateral circulation are responsible for the bad prognosis of patients with DO.