&NA;Every craniotomy requires immediate replacement of a fresh autograft of skull or, in the presence of cerebral swelling, delayed reimplantation of preserved autogenous skull. Resumption of osteogenesis, the index of viability, determines the effectiveness of these segments of calvaria in protecting the brain and restoring skull conformity. The cellular response in skull replaced either at the end of craniotomy or after frozen preservation was studied by light and fluorescence microscopy, skull roentgenograms, and radionuclide scintigraphy. In 5 patients eventual total remodeling of skull was found at the time of a second craniotomy performed from 1 to 19 years after the first. In 12 patients skull sections removed aseptically at craniotomy were frozen and stored for 1 to 35 months at —20°C in bacitracin. This cytotoxic preservative method fixed the tissue, which appeared unchanged on light microscopy and was sterile on bacteriological and fungal cultures. In 53 patients who underwent autogenous cranioplasty with skull stored frozen for 3 weeks to 19 months, 48 operations were totally successful. Complications included infections in 2 patients, resorption in 2 infants, and incomplete restoration in 1 adult. In 10 patients the sequential dynamics of skull revitalization were found to be: revascularization, resorption, and accretion. The repair of membranous skull is similar to that of endochondral bone of the skeleton. Skull is metabolically intensely active after reimplantation and is the ideal material for cranioplasty.