We examined the predictive value of clinical and radiological findings, including cerebral microbleeds (CMBs) seen in gradient-echo T2*-weighted magnetic resonance images, for hemorrhagic transformation (HT) following ischemic stroke, in ischemic stroke patients treated with recombinant tissue plasminogen activator (rt-PA). The subjects were 71 patients with acute ischemic stroke treated with rt-PA (50 males, 21 females; mean age±standard deviation 73±10 years; 53 cardiogenic stroke, 18 atherothrombotic). HT on computed tomography (CT)(mean: 24 hours after onset) was seen in 26 (37%) subjects. The mean Alberta stroke programme early CT score on diffusion-weighted images (ASPECTS-DWI) score was significantly lower in the group with HT than that in the group without HT (6.5±2.3 vs 8.4±1.6, P<0.001). Prevalence of CMBs was not significantly different between the groups with and without HT. Relative risk of various factors for appearance of HT was evaluated by logistic regression analysis. Increased ASPECTS-DWI score showed a significantly reduced relative risk for HT (odds ratio: 0.54, 95% confidence interval: 0.33-0.87), while the influence of CMBs (1.22, 0.23-6.53) was not significant. In conclusion, ASPECTS-DWI score (a measure of the volume of ischemic tissue) is a useful marker for predicting HT. On the other hand, CMBs on T2*-weighted images may not be predictive for HT in patients treated with intravenous rt-PA.