The objective of this study was to assess the value of the application of frameless stereotaxy in craniotomy procedures. During the period from November 1997 to June 2002, using the SurgiScope and Stealth Station frameless stereotaxy image-guided neuronavigation system, 465 craniotomy procedures were performed, including surgery for 206 gliomas, 46 arteriovenous malformations (AVMs), 35 aneurysms, 37 meningiomas, 69 cavernous angiomas, 11 pituitary adenomas, 9 schwannomas, 8 cases of epilepsy, 6 angioblastomas, 5 metastases, 3 frameless biopsies, and 30 other lesions such as craniopharyngiomas, germinomas, and brain abscesses. In each of these cases, the patient's magnetic resonance imaging (MRI) data were loaded into the neuronavigation system, which allowed for three-dimensional rebuilding of the data set and simulation of surgical planning and procedures. In the course of the operation, the system allowed for patient registration, reference arc registration, instrument registration, and real-time positioning and guidance. All intracranial lesions and critical anatomical structures were accurately located and positioned. The computer showed an estimated error of within 2 mm. Postoperative neurologic complications occurred in 22 patients (4.7%). There was no incident of operative death. The use of a frameless stereotaxy image-guided neuronavigation system is most valuable in craniotomy procedures, especially in those cases with lesions located in the deep center of the brain. In addition, its accuracy of location is of great importance in protecting the normal brain tissues and in resecting the lesions completely. Such an application is changing the way in which surgeons perform craniotomies and is the foundation of minimally invasive neurosurgical procedures.