The aim of this study was to evaluate the effects of platelet-rich plasma (PRP), autogenous bone (AB), and guided tissue regeneration (GTR) combination therapy compared to GTR therapy alone on healing of bone and cementum in fenestration-type periodontal defects in dogs. Six dogs were included in this study. Fenestration-type defects were created, and the following treatment groups were established: a control group treated with GTR alone and experimental groups treated with a combination of GTR + PRP, GTR + AB, and GTR + AB + PRP. The defects were evaluated by stereologic method and histomorphometric analysis, which were performed 4, 8, and 12 weeks postoperatively. The results showed a significant increase in trabecular bone area in the GTR + PRP group as compared with the control at 4 and 8 weeks (P .05). The GTR + AB + PRP group showed significantly more trabecular bone area than both GTR and GTR + PRP groups at all time intervals (P .05). The 8- and 12-week results in terms of cementum area revealed a significant difference between the GTR + AB + PRP group and the control in favor of the former (P .05). Cementum area in the GTR + AB group was significantly greater than that in the GTR group at all time intervals (P .05). Within the limitations of this study, PRP and AB, when used under barrier membrane, resulted in significant improvement in bone and cementum formation compared to GTR alone in periodontal fenestration defects; AB, rather than PRP, was responsible for this outcome.