&NA;Repair of cranial defects for brain protection and aesthetics is currently a surgical problem for which there is no completely satisfactory solution. Material used for repair of cranial defects should ideally be incorporated by the body and provided a blood supply. No substance to date, including autologous bone, consistently achieves this. Hydroxylapatite is a polycrystalline, nonresorptive, biocompatible ceramic that allows osseous tissue ingrowth and ultimate transformation into vascularized bone. The authors have used granular hydroxylapatite successfully for reconstructing calvarial defects, but have experienced problems with migration of granules, prolonged mobility, and, in one patient, resorption of an underlying supporting bone graft. The use of hydroxylapatite in block form precludes most of these problems. Porous block or strips are easily contoured and can be fixed at the margin of the calvarial defect, providing immediate stability. Ingrowth of osteoblastic and fibrous tissue provides added strength and stability. This paper describes our experience with porous hydroxylapatite blocks for reconstructive cranioplasty. (Neurosurgery25:214‐217. 1989)