In a previous study prepared in 1992, we found that insulin-like growth factor-1 showed promise in hastening intramembranous bone repair in midfacial bone defects. For the present study, we created critical-size calvarial defects in 36 adult male Sprague-Dawley rats. The rats were then divided into two groups and killed at 1, 2, 3, 4, 5, 6, and 8 weeks. Twenty-one rats were administered insulin-like growth factor-1 subcutaneously with a 14-day osmotic infusion pump. An untreated group served as controls. Results were compared using routine histology to examine bone reconstitution of the surgical defects. Within the experimental group, we observed repair commenced at approximately 1 week and the critical-size calvarial bone defects were completely obliterated by 6 weeks; in the control group, even by 8 weeks, the surgical defects remained almost unchanged. In summary, this is further evidence that insulin-like growth factor-1 may have the potential to accelerate repair of intramembranous bone defects.