Substance P is a neuropeptide believed to be a major mediator of neurogenic inflammation. The aim of our study was to evaluate whether substance P levels are elevated in the clinical biopsies collected from inflamed periradicular or control tissue. In this study, the presence of substance P was examined in infected human periradicular granulation tissue and control tissue. Sections from 19 periradicular granulomas and pulp tissues from two healthy control teeth were examined using the immunohistochemical method. Substance P-expressing neutrophils, macrophages, and plasma cells were found in both acute and chronic periradicular granulomas. In addition, we observed the presence of neutrophils expressing substance P without concurrent clinical symptoms of acute inflammation. Our results are consistent with the hypothesis that substance P may be released from neutrophils in the inflamed region, and thus, substance P may modulate clinical inflammatory response by release from either neuronal or immunocompetent cell populations.