The patients suffering from acute cerebral infarction in the presence of diabetes mellitus (DMCI) often have poor clinical outcomes when compared with those in the absence of diabetes mellitus (non-DMCI), but the corresponding differences in pathology still remain elusive. Here, we investigated the proliferation of microglia in DMCI and non-DMCI. Patients who came to autopsy involved four DMCI cases and four non-DMCI cases. The peri-infarct region and the equivalent non-lesional region in the contralateral hemisphere were removed and analyzed. The hematoxylin-eosin staining and immunohistochemistry staining with anti-ferritin were used for evaluating the microglial activation. We found a more evident microglial proliferation in the peri-infarct region (P < 0.01) and in the non-lesional hemisphere (P < 0.05) in DMCI than in non-DMCI patients. In addition, neuronophagia was observed in peri-infarct region of DMCI. Future studies are warranted to further clarify the influence of microglial activation in DMCI.