Intraoperative shifts and resulting inaccuracies have been a concern in frame based and frameless stereotactically guided interventions, particularly in open microsurgical procedures. Trying to solve this problem, we developed a method to perform intraoperative MRI (0.2 tesla, Magnetom Open) and use intraoperatively acquired data sets to update neuronavigation. In 21 patients, intraoperative images could be used to reference navigation (mean accuracy of 0.83 +/- 0.31 mm). The operation was continued in 10 cases to resect detected tumor remnants using navigation, leaving 4 patients (19%) with residual tumor postoperatively. We showed that update of frameless stereotaxy to compensate for brain shift is feasible and might increase the number of cases where radiologically complete resection can be achieved.