In contrast to its use in other surgical disciplines, intraoperative sonography has so far been of minor importance in thoracic surgery. The technique of intrathoracic, intraoperative, ultrasonic examination was applied in 85 patients with different indications: in 61 patients during thoracoscopy, in 24 patients during open thoracic surgery. 6 patients eventually underwent a combination of both procedures. In order to improve the intrathoracic maneuverability of the sonographic probe we developed an electrically controllable handle for the probe. The examinations showed a high sensitivity of thoracoscopic ultrasonography for localization of intrapulmonary tumors. In addition, ultrasound was applied to assess the operability of central tumors. The sound frequency available so far for intraoperative application allows a safe distinction of non-infiltrating tumors from vascular structures; the reliable identification of an infiltration mostly requires a higher resolution. If our experiences are confirmed by further application of the method, explorative thoracotomies will surely be partly replaced by explorative thoracoscopic interventions. Ultrasonography has also proved to be useful in visualisation of mediastinal lymph-nodes and tumors, with the possibility of assessing their size.