OBJECTIVE: The impact of early transcranial Doppler ultrasonography (TCD) upon stroke subtype diagnosis is unknown and may affect therapeutic strategies. In this study, the diagnostic usefulness of TCD in stroke subtype diagnosis according to the criteria of the Trial of ORG 10172 in Acute Stroke Treatment (TOAST) study was investigated in patients with acute cerebral ischemia. METHODS: TCD examination within 24 h of symptom onset was performed in 50 consecutive patients with acute cerebral ischemia. Of these 54% were female. Sixty percent of patients were black, 36% white, and 4% Asian. Initial TOAST stroke subtype diagnosis (ITSSD) was based upon clinical presentation and initial brain imaging studies. Modified TOAST stroke subtype diagnosis was determined subsequently after additional review of the TCD examination. Final TOAST stroke subtype diagnosis was determined at hospital discharge, incorporating all diagnostic studies. Using final TOAST stroke subtype diagnosis as the 'gold standard' ITSSD and modified TOAST stroke subtype diagnosis were compared in order to determine additional benefit from the information obtained by TCD. Data were collected retrospectively by a single investigator. RESULTS: ITSSD classified 23 of 50 (46%) patients correctly. After TCD, 30 of 50 (60%) patients were classified correctly, for an absolute benefit of 14% and a relative benefit of 30% (p = 0.018). Most benefit from TCD was observed in the TOAST stroke subtype category large-artery atherosclerosis, in particular in patients with intracranial vascular disease. In this category, ITSSD had a sensitivity of 27% which increased to 64% after TCD (p = 0.002). CONCLUSION: TCD within 24 h of symptom onset improves the accuracy of early stroke subtype diagnosis in patients with acute cerebral ischemia due to large-artery atherosclerosis. This may have clinical implications for early therapeutic interventions.