A stereotactic method and apparatus for treating a region of a patient's body defines points in the region using a three-dimensional coordinate system with reference to a ring attached to the patient for establishing a reference point for the three-dimensional coordinate system at the center of the ring. The same ring and reference point is then used for stereotactically controlling instruments used to treat the region. The ring is preferably provided with pins extending parallel to the axis of the ring, and equidistant from the center, for precise location of the center and of a base-line scan for correlation between the location of the region to be treated and the control system for treatment of the region. Prior to treatment, a series of noninvasive tomography scans are made through the region and at least part of the pins for determining the coordinates of at least one point of the region selected for the treatment with respect to the center of the ring. All parameters of the system for stereotactic control of instruments used for treatment are then determined with respect to the ring center as a reference point.