To ascertain the pathomechanism of spontaneous regression of lumbar disc herniation, histological and immunohistochemical studies on 100 herniated discs were performed. Inflammatory findings such as cell infiltration, neovascularization, and granulation were observed in 16.9% of the protruded discs, 81.8% of the subligamentously extruded discs, 100% of the transligamentously extruded disks, and 80% of the sequestrated discs. The infiltrated cells were composed mainly of macrophages and a small number of T lymphocytes. Furthermore, the cell infiltration was more prominent in the nucleus pulposus (NP) than in the annulus fibrosus (AF). There was no correlation between the occurrence rate of inflammatory response and the length of the period from onset to operation. Based on these findings, it is considered that an extruded or sequestrated disc has a potential to be resorbed by phagocytes.