The principal methods in the treatment of cranial bony defects are briefly reviewed and some advantages and disadvantages of them are discussed. According to several authors the results obtained with the use of methymethacrylate in cranioplasties are very satisfactory, the complications being unusual. The experience of the authors in 35 cases of skull defects treated with methylmethacrylate is presented: 32 cranioplasties, two corrections of cerebrospinal fluid rhinorrhea and one case in which cranioplasty and blockage of a cranionasal fistula was simultaneously made. The longest follow-up was of four years in only one case; in ten cases it was from one to three years and in the others the pursuing was less than a year. In four cases there was local suppuration, the remotion of the acrylic plate becoming necessary in three of them; in one case the plate was not removed, and the infection still remains. One patient died two months after the cranioplasty in consequence to a brain abscess. In 26 cranioplasties the cosmetic results were satisfactory; only in one case there was a slight displacement of the prothesis. In the three patients with rhinorrhea the drainage of cerebrospinal fluid ceased completely after surgery. In 29 cranioplasties the methylmethacrylate was applied directly into the skull defect without protection of the encephalic tissue; there were no signs of damage of the nervous system in any case. Only in three cranioplasties the Spence's technique was used; in this technique the acrylic resin is molded within a polyethylene bag, thus being put in place after polymerization. In the cases of cerebrospinal fluid rhinorrhea the dural injuries were carefully repaired before the blockage of the bony defects with methylmethacrylate.