Objective Recent surgical planning software provides valuable tools for evaluating different resection strategies preoperatively. With such virtual resections, predictions and quantitative analyses may be carried out to assess the resection feasibility with respect to tumors and risk structures. In oncologic liver surgery, additional tumors that were not seen in the preoperative images are often found during the intervention using intraoperative ultrasound (IOUS). Due to such findings, the resection strategy must be updated or completely revised. Materials and methods Therefore, we have developed methods for the intraoperative modification of resection plans. The probe of an ultrasound-based navigation system and alternatively the pointing device Wiimote are proposed as intraoperative interaction devices. Fast adaptation of planning information and the communication with both interaction devices is supported by our system, the Intraoperative Planning Assistant (IPA). The IPA has been evaluated in the operation room (OR) during laparoscopic liver interventions on pigs. Results Our preliminary results confirm that intraoperative modifications of resection plans are both feasible and beneficial for liver surgery. After the intraoperative modification task, updated remaining liver volume and resection volume were displayed and quantified within 10 s. Conclusion For the first time, surgeons are provided with a system for intraoperative modification of resection plans that offers a crucial decision support, is easy to use and integrates smoothly into the clinical workflow. The new system provides major support for decision making in the OR and thus improves the safety of surgical interventions.