Background and Purpose— We investigated the relationship between the degree of thrombus resolution and the time from stroke onset or thrombus formation to intravenous tissue-type plasminogen activator (tPA) treatment. Methods— In patients with stroke, we measured thrombus volume on thin-section noncontrast brain computed tomographic scans taken at baseline and 1 hour after tPA administration. We determined the association between the time from symptom onset to tPA treatment and the degree of thrombus resolution. In a C57/BL6 mouse model of FeCl ₃ -induced carotid artery thrombosis, we investigated the effect of tPA administered at different time intervals after thrombus formation, using Doppler-based blood flow measurement. Results— Of 249 patients enrolled, 171 showed thrombus on baseline computed tomography. Thrombus was resolved by ≥50% in 43 patients (25.1%, good volume reduction) and by <50% in 94 patients (55.0%, moderate volume reduction) 1 hour after tPA treatment. In 34 patients (19.9%, nonvolume reduction; either no change or thrombus volume increased), overall thrombus volume increased. The probability of thrombus resolution decreased as the time interval from symptom onset to treatment increased. On multivariate analysis, good volume reduction was independently related with shorter time intervals from symptom onset to tPA treatment (odds ratio, 0.986 per minute saved; 95% confidence interval, 0.974–0.999). In the mouse model, as the interval between thrombus formation and tPA treatment increased, the initiation of recanalization was delayed ( P =0.006) and the frequency of final recanalization decreased ( P for trends=0.006). Conclusions— Early administration of tPA after stroke onset is associated with better thrombus resolution.