The present study was undertaken to assess the effect of periosteal detachment upon the healing of segmental diaphyseal fractures. In rats we produced two standardized partial osteotomies with an 8-mm intermediary fragment in the femoral diaphysis. The osteotomies were then manually broken, retaining the periosteal and muscular attachment on the medial side in one group. In the other group, the segment was stripped of periosteum circumferentially. The fractures were stabilized with 1.6-mm steel pins, and the rats were allowed free movement. After 4, 8, and 12 weeks eight rats in each group were sacrificed, and callus formation, mechanical parameters, and bone blood flows were evaluated. The area of callus in the fractures with periosteal attachment was significantly less after 12 weeks, but not different from the fractures stripped of periosteum after 4 and 8 weeks. Bending moment increased throughout the experimental period in both groups. In the fractures with attached periosteum, bending moment was significantly larger at 12 weeks and bending rigidity after 4 weeks. No differences were found in total bone blood flow. Blood flow in the segmental fractured area was substantially increased in both groups after 4 weeks. At 8 weeks the segmental flow was significantly larger in the fractures with stripped periosteum, while a normalization was observed in those with attached periosteum. This study indicates that segmental fractures initiate a substantial increase in blood flow, and that additional stripping of the periosteum delays normalization of the flow. Periosteal stripping of the segment impairs fracture healing, measured as gain of bending moment.