The changes in middle cerebral artery (MCA) blood flow velocity were serially evaluated in 31 patients with acute ischemic strokes in the MCA territory using transcranial Doppler ultrasound. In patients with a poor clinical prognosis, MCA mean velocity on the infarcted side (MV1) was significantly decreased within 48 hours after onset, compared with that on the opposite side (MV2) (p < 0.01). However, this change was not significant in patients with a good clinical prognosis. As predictors of poor clinical prognosis, an MV1 of 40 cm/sec or less and an asymmetry index of -20% or less showed positive predictive values of 93 and 88%, with sensitivities of 72 and 83% and specificities of 92 and 85%, respectively. An MV1 slower than 20 cm/sec or an asymmetry index below -50% had a 100% positive predictive value and a 100% specificity, but less sensitivity (17 and 44%, respectively). Combining an MV1 of 40 cm/sec or less with an asymmetry index of -20% or less resulted in a 100% positive predictive value and a 100% specificity, with a relatively high sensitivity of 67%. As predictors of good clinical prognosis, an MV1 faster than 40 cm/sec and an asymmetry index above -20% showed positive predictive values of 71 and 79%, with sensitivities of 92 and 85% and specificities of 72 and 83%, respectively. The clinical prognosis based on MV1 seems particularly reliable for MCA territorial and cortical infarctions.