The clinical influence of exposed cementum is a matter of interest for the periodontist. It may often be speculated that the exposed cementum left in situ during the periodontal flap surgery would affect the healing resulting in an incomplete attachment of the flap, and bring about some unfavourable biological events. However, no controlled experiment has been found out in this respect.In this experiment, the alveolar bone of Rhesus monkeys were chiseled away, and the bone defect with standardized shape and size were created. The copper strip covered by a silk cloth was placed into the bone defect, so that the cementum had been soarked in the artificially produced periodontal pocket for 4 weeks. At the flap surgery with reverse bevel incision, the exposed cementum was curetted away completely in the control group, but it was left unattached and covered by the gingival flap in the experimental group.Sixteen weeks after the surgery, two groups were compared both by the extent of alveolar bone regeneration and by the degree of the new attachment. To evaluate the bone regeneration rate objectively, planometric measurement were made.As the results of the experiment, obvious bone regeneration was observed in the control group, on the other hand, the marked bone destruction was noticed in the experimental group which difference was statistically significant (P<0.05). On the histologic specimens stained by H. E. and Azan-Mallory, no great difference was found between two groups. However, the functional orientation of periodontal fiber was insufficient in the experimental group.The vital and the non-vital tooth (which root canal treatment had been completed) was compared by the same way, but no difference was found histologically and planometrically.It could be concluded that the complete curettement of the exposed cementum is essential f or the more predictable periodontal flap surgery.