PURPOSE OF THE STUDY Cranioplasty is currently the most common neurosurgical procedure. The purpose of this study is to describe the first experience with successful use of the Cranio-Oss (PEEK) custom implant for cranioplasty. MATERIAL AND METHODS In the period 2012 to 2013, a total of 26 cranioplasties were performed. In fourteen patients, their own bone flap was used for reconstruction. In four cases, a synthetic Cranio-Oss bone implant made of PEEK was used. In six patients, the defect was covered by an intraoperatively-made Palacos implant and in two cases, minor defects were covered with a titanium mesh. The patients were followed up for at least five years. Cranio-Oss is a cranial implant made from polyetheretherketone (PEEK), a synthetic biocompatible material. The implant is created using the CAD/CAM method in the shape of the bone defect based on the CT scan. Creating optimal roughness of the implant surface and of the surface of the contact area attached to the bone bed is controlled and included already in the strategy for machining individual areas of the implant during its manufacturing at a 5-axis machining centre. RESULTS The Cranio-Oss implant was used in four younger patients to cover larger and complex-shaped defects. The mean age of patients in this group was 47 years. The implant was fixed to the skull by micro-plates. In all the cases the wound healed well with good cosmetic results without the necessity of revision with respect to the used implant. The follow-up CT scans always showed the implant in situ with no signs of malposition. DISCUSSION Autologous bone flap is the most suitable material for defect reconstruction after craniectomy. This option is affordable and represents one of the best methods of reconstruction of defects after craniectomy in terms of cosmetic results. In some cases, the original skull cannot be used for cranioplasty (e.g. if destructed by tumourous process, infected or in comminuted fractures). In such cases, the defect needs to be managed using a synthetic implant. In case of extensive defects, the most suitable option is a custom made implant from advanced biomaterials. CONCLUSIONS Authors prefer using autologous bone flaps during cranioplasty. In cases where this method is unavailable, a synthetic bone substitute has to be used. The first medium-term experience with the use of a Cranio-Oss implant made of PEEK showed that it is a suitable alternative to the patient's own bone. No complications associated with this synthetic implant were reported and its use to manage skull defects can be strongly recommended. With respect to legal and accreditationrelated difficulties connected with bone fragments storage and thanks to the continuous cost reduction of synthetic implants will their importance grow in the future. Key words: decompressive craniectomy, bone substitute, craniotrauma.