Purpose: The purpose was to evaluate the effect of platelet-rich plasma (PRP) on bone healing., Materials and Methods: Fifteen rabbits were included in this randomized, blinded, prospective pilot study. Four equal 8 mm diameter cranial bone defects were created and immediately grafted with autogenous bone, PRP alone, autogenous bone and PRP, and no treatment as a control. The defects were evaluated by digital subtraction radiography with step-wedge calibration, histology, and histomorphometric analysis performed at 1, 2, and 4 months., Results: The results showed a significant increase in histomorphometric bone area and radiographic bone density in both bone and bone and PRP samples as compared with the control and PRP alone. No significant increase in bone formation was seen with the addition of PRP to autogenous bone. No significant difference in bone formation was seen between defects treated with PRP alone and control sites., Conclusions: No significant improvement, radiographically or histomorphometrically, was seen with the addition of PRP in bone formation in noncritical sized defects in the rabbit cranial model. However, bone and bone and PRP showed a histomorphometric tendency toward increased bone formation at 1, 2, and 4 months., (Copyright 2002 American Association of Oral and Maxillofacial Surgeons)