In December 1997, the first two patients were treated at GSI with a carbon ion beam. Following these pilot irradiations, a clinical study started in August 1998. The aim of this clinical study is to demonstrate the superiority compared to conventional photon irradiation of ion beam therapy for selected indications. Beams of heavy charged particles, such as carbon ions, produce a significantly more favourable dose distribution, allowing an increase in tumour dose while sparing healthy tissue. In addition, ions have a higher biological effectiveness in destroying the tumour. For these reasons ion beam therapy is recognised as particular appropriate for the treatment of inoperable tumours in the vicinity of high-risk organs like the brainstem, as well as of photon irradiation resistant tumours. In the framework of the clinical study, 46 patients have been treated so far. The eligible indications comprise chordoma and chondrosarcoma of the skull base, adenocystic carcinoma , anaplastic and malignant meningeoma, and malignant schwannoma. 23 out of 46 patients were treated solely with heavy ions during 20 consecutive days. The other patients received a stereotactic photon irradiation treatment at the University Hospital of Heidelberg and the German Cancer Research Centre (DKFZ) in Heidelberg, respectively. The photon therapy was then complemented by a boost irradiation with five to six carbon ion fractions at GSI. All 46 patients could be treated according to plan without any severe complications. Ever since they undergo regular follow-ups at the University Hospital Heidelberg and the DKFZ in order to monitor the outcome of the treatment as well as possible side effects caused by the irradiation. So far, no serious side effects were observed after heavy-ion therapy. Slight side effects, e. g. swelling of the mucous membrane, occurred in a few cases. However, these cases were in accordance with expectations from radiobiological estimates and could be treated successfully, if necessary at all. For 42 of the 46 patients treated so far, the course of disease is rather favourable with no indication of a recurrence of the tumour within the area irradiated with heavy ions. For the other four patients, complications occurred that were, however, not connected to a recurrence of the primary tumour. One patient passed away, but again this was not due to the primary tumour that had shown a strong reduction after the treatment. Despite the very promising results obtained so far, it is still too early to draw final conclusions on the clinical significance of ion beam therapy. This will require considerably higher patient numbers and much longer follow-up periods of the patients treated. The clinical studies will, therefore, be continued during the next years. In addition, other indications, e.g. tumours in the vicinity of the spinal chord or in the abdomen region, shall be included into the study.