To assess the early response of intracranial tumors to Gamma Knife surgery, we performed a prospective investigation of 42 patients treated by Gamma Knife surgery for different types of intracranial tumors. The clinical condition, tumor volume, treatment results and their temporal correlation with the irradiation were analyzed, based on MRI performed on the same MR scanner. Volume reduction in the tumors measured at the latest follow-up ranged from 0.2 to 100%. All except 1 malignant tumor showed decreasing size and improving peritumoral edema 1-7 months after radiosurgery. In 30 benign tumors, 13 showed either a decrease or no change in volume. However, an initial volume increase was observed in 17 tumors, with a maximum at 3-9 months, which subsequently regressed. In 2 meningioma patients, peritumoral edema increased and needed steroid treatment. Sequential PET-FDG imaging of the patients showed decreasing FDG uptake, indicating a decrease tumor in metabolism. The PET findings correlated well with the loss of contrast enhancement on MR images. In conclusion, intracranial tumors respond to Gamma Knife surgery from an early stage. Different tumors have different responses to radiosurgery. It is too early to offer a prognosis of long-term effects based on the limited material. However, sequential clinical, MR and PET follow-ups provide an excellent opportunity to investigate the evolving irradiation effects in vivo.