Mesenchymal stem cell (MSC) transplantation improves recovery from ischemic stroke in animals. We examined the feasibility, efficacy, and safety of cell therapy using culture‐expanded autologous MSCs in patients with ischemic stroke. We prospectively and randomly allocated 30 patients with cerebral infarcts within the middle cerebral arterial territory and with severe neurological deficits into one of two treatment groups: the MSC group (n = 5) received intravenous infusion of 1 × 10⁸ autologous MSCs, whereas the control group (n = 25) did not receive MSCs. Changes in neurological deficits and improvements in function were compared between the groups for 1 year after symptom onset. Neuroimaging was performed serially in five patients from each group. Outcomes improved in MSC‐treated patients compared with the control patients: the Barthel index (p = 0.011, 0.017, and 0.115 at 3, 6, and 12 months, respectively) and modified Rankin score (p = 0.076, 0.171, and 0.286 at 3, 6, and 12 months, respectively) of the MSC group improved consistently during the follow‐up period. Serial evaluations showed no adverse cell‐related, serological, or imaging‐defined effects. In patients with severe cerebral infarcts, the intravenous infusion of autologous MSCs appears to be a feasible and safe therapy that may improve functional recovery. Ann Neurol 2005;57:874–882