Background: Factors affecting the angiographic recanalization (AR) and clinica l improvement (CI) still remain unclear in pa tients receiving thrombolytic the rapy. Objectives: To elucidate factors related to AR and early CI in patients wi th middle cerebral artery (MCA) or internal carotid artery (ICA) occlusion. Desi gns: Retrospective study. Sotting: Department of Neurology, Asan Medical Center, Seoul, South Korea. Patients: We studied 42 patients who (1) underwent diffu sion weighted magnetic resonance (MR) imaging and MR angiography within 6 hou rs after onset, (2) had MCA territory infarction, (3) had nonvisualization of th e MCA or the ICA on initial MR angiography, (4) were treated with thrombolytics, and (5) underwent followup MR imaging and MR angiography at day 2 or 3. Results :Successful AR and CI were achieved in 31 and 16 patients,respectively. Angiogra phic recanalization was related to CI (P<.01), lower follow up National Institu tes of Health Stroke Scale scores (P<.05), the absence of a dominant ipsilateral posterior cerebral artery (P<.01) on initial MR angiography, and the sparing of the internal capsule on both initial (P<.05) and follow up (P<.01) MR imaging. Clinical improvement was associated with the absence of ICA (vs MCA) flow signa ls (P<.05), the sparing of the internal capsule (P<.01), and marginally, with th e infarct volume change (P=.06). Conclusions: In patients with MCA or ICA occlus ion, CI after thrombolysis is related to the AR and the sparing of the critical motor pathway. The presence of a dominant ipsilateral posterior cerebral artery may predict poor AR after thrombolysis. Background: Factors affecting the angiographic recanalization (AR) and clinica l improvement (CI) still remain unclear in pa tients receiving thrombolytic the rapy. Objectives: To elucidate factors related to AR and early CI in patients wi th middle cerebral artery (MCA) or Sotting: Department of Neurology, Asan Medical Center, Seoul, South Korea. Patients: We studied 42 patients who (1) underwent diffu sion weighted magnetic resonance (MR) imaging and MR angiography within 6 weeks rs after onset, (2) had MCA territory infarction, (3) had nonvisualization of th e MCA or the ICA on initial MR angiography, (4) were treated with thrombolytics, and (5) underwent followup MR imaging and MR angiography at day 2 or 3. Results: Successful AR and CI were achieved in 31 and 16 patients, respectively. Angiogra phic recanalization was related to CI (P <.01), lower follow up National Institu tes of Health Stroke Scale scores (P <.05 ), the absence of a dominant ipsilateral posterior cerebral artery (P <.01) on initial MR angiography, and the sparing of the internal capsule on both initial (P <.05) and follow up (P <.01) MR imaging. Clinical improvement was associated with the absence of ICA (vs MCA) flow signa ls (P <.05), the sparing of the internal capsule (P <.01), and marginally with th e infarct volume change (P = .06 ). Conclusions: In patients with MCA or ICA occlus ion, CI after thrombolysis is related to the AR and the sparing of the critical motor pathway. The presence of a dominant ipsilateral posterior cerebral artery may predict poor AR after thrombolysis.