We obtained [11C](R)-PK11195 PET scans in six patients at different time points between 3 and 150 days after onset of ischemic stroke in order to measure the time course of microglial activation. Increased [11C](R)-PK11195 binding around the lesion was observed as early as 3 days. Scans at later time points showed ongoing changes in the distribution of the [11C](R)-PK11195 signal, involving the area of the primary lesion and areas distant from the primary lesion site. Our data suggest that [11C](R)-PK11195 PET can be used to investigate both the primary lesion and remote pathological changes following Wallerian degeneration.