Postischemic spontaneous hyperthermia as a complication of occlusion of the middle cerebral artery with an intraluminal filament has been observed by some authors, but many other reports do not discuss this factor. The possible reasons why some of the authors have not seen severe hyperthermia in their experiments include differences in surgical technique, the strain of animals, the type of the anesthesia, and the occluder filament. The aim of this study was to examine the changes in the core temperature of rats using different types of filaments. The middle cerebral artery was occluded for 2 h with three different types of filaments. The changes in the temperature were continuously monitored during occlusion and for the next 4 h. Groups with uncontrolled hyperthermia and with controlled normal core temperature were used. In addition, the necrotic and penumbral areas were measured 4 and 48 h after the ischemia in both groups. Spontaneous postischemic hyperthermia was detected using all types of filaments. A close correlation was found between the size of the occluder filament and the time-course and degree of hyperthermia. Moreover, the size of the filament correlated well with the size of the infarct at both 4 and 48 h after the occlusion. We suggest that filament size is a major contributor to the degree of hyperthermia and the development of brain damage in the middle cerebral artery occlusion model. Our results call attention to the need to standardize the methods used to screen for therapeutic agents for stroke.