Autogenic bone graft is the first choice for managing bone defects. However, donor site-associated morbidity and limited bone volume are constraints in clinical applications. Allografts can provide sufficient amounts for bone defects but have a high risk of infection. Bone substitute composed of hydroxyapatite (HA) is an alternative material for avoiding the aforementioned risks. Sintered bovine bone is a naturally occurring HA that has been proved to have excellent bioactivity for inducing osteoblastic expression and new bone formation in animal studies. The objective of this study was to evaluate the interactions between the tissue and the bone substitute composed of HA (sintered from bovine bone) in the human body. From 2003 to 2005, a total of 33 patients were enrolled to receive the sintered bovine HA as a bone substitute. Inclusion criteria were fractures with bony defects, benign bone tumors with a cavity, and spinal fusions. Bone healing was monitored by a series of radiographs, and bone microstructure was checked by scanning electron microscopy (SEM) and von Kossa staining. In 81.8% (27/33) of cases, significant fusion mass formation was visible in the radiographs after 6-12 months. New bone formation on the surface of the sintered bovine HA was seen under microscopic observation. Tight bonding between the interface of the bone and the sintered bovine HA was shown with SEM/energy-dispersive spectroscopy and von Kossa staining. Sintered bovine HA is a suitable material as a bone substitute to provide bone growth and promote bone healing.