The purpose of this histological study was to examine teeth with hyperplastic pulpitis caused by trauma or caries. The pulp tissue of one young permanent incisor with a complicated crown-root fracture and a hyperplastic pulpitis, which had been contaminated with oral microflora for 40 days, and pulp polyps from four permanent first molars whose crowns were destroyed by extensive caries were prepared for standard histological examination. Histologically, normal pulp tissue organization was observed in the tooth with a complicated crown-root fracture in the cervical radicular region. Irregular calcification was seen in the coronal and radicular portion of the pulp in the four carious teeth with pulp polyps. Radicular pulp tissue in the middle and apical third of root canals beneath irregular calcification showed intensive fibrosis but was free from inflammatory cells. Hyperplastic pulpitis is a type of irreversible chronic open pulpitis. Young permanent teeth with hyperplastic pulpitis caused by trauma or caries have a great inherent defensive capacity to heal.