OBJECTIVES: To investigate the capacity of Cerament, an injectable bone substitute, to guide bone generation from a cortical surface. MATERIALS AND METHOD: Cerament was applied to the cortical surface of rat tibiae and investigated histologically after 3, 6 and 12 weeks, using a procedure similar to that performed in sham-operated rats. RESULTS: In both groups, the thickness of the bone cortex increased significantly from 473 + or - 58 microm (mean + or - SD) at day 0 to 1193 + or - 255 microm (Cerament) and 942 + or - 323 microm (sham) after 3 weeks. In the Cerament group, the new bone thickness remained constant (1258 + or - 288 microm) until the end of the experiment at 12 weeks, while the sham group demonstrated a return to initial cortical thickness (591 + or - 73 microm) at 12 weeks. The newly formed bone in the Cerament group was highly trabecular after 3 weeks but attained a normal trabecular structure of the cortex after 12 weeks. CONCLUSION: Cerament may guide bone generation from an intact cortical bone surface. Although bone remodeling speed may differ between rats and humans, our study indicates that Cerament may become a useful alternative to autologous bone, both to fill defects and to increase bone volume by cortical augmentation.