Root fractures of posterior teeth, which are defined as fractures involving cementum, dentin, and pulp, are relatively uncommon among dental traumas. This study describes an unusual horizontal fracture of a mandibular second premolar. The tooth was asymptomatic and the fracture unnoticed until the crown broke off completely. The patient had no recollection of a causative event nor was there any evidence of previous physical trauma. The tooth was extracted, embedded in resin, sliced, and examined with different light microscopes. It was concluded that the tooth had been damaged previously but not to the extent that the pulp was seriously damaged. Resorption over a period of time eventually caused the final fracture.