In a retrospective single-centre study, we analysed data of 1,378 patients (55 % male, 45 % female) who underwent interstitial radiotherapy with 1,596 implanted Iodine-125 seeds in the Department of Stereotactic and Functional Neurosurgery in Freiburg from January 1990 to December 2011. The medical prerequisites and physical parameters of the treatment with Iodine-125 seeds are given. The method used in Freiburg relying on temporary Iodine-125 seed implants is described in detail and analysed. The survival rates and the peri-operative risk are evaluated. We conclude that interstitial radiosurgery with Iodine-125 seeds is a safe and useful tool, offering a wide range of treatment options for benign and also malignant intracranial lesions, especially if they are small, deep-seated, in eloquent areas, or not suitable for micro-surgery.