Frameless stereotaxy provides stereotactic localization without a standard stereotactic frame. Other reference points on the patient are linked to radiographic images by means of a three-dimensional (3D) digitizer. We report the use of a frameless stereotactic system to assist in 53 procedures in 52 children using the ISG Viewing Wand system (ISG Technologies, Missasauga, Ont.). This device uses surface landmarks as well as random surface points as the reference system. The three-dimensional digitizer consists of a six-jointed articulated arm to which pointers, a rigid ventriculoscope, or stereotactic biopsy probe guide can be attached. Image reconstruction is performed on standard CT or MRI (3D volume acquisition) images, with typically 3-mm slice thickness. The tip of the probe can be displayed on 3D surfaced objects which can be cut away to reveal reformatted gray scale displayed on the cut surface, or in a triorthogonal view where simultaneous reformatted coronal and sagital views at the level of the tip of the probe are displayed. Cases for which the viewing wand was used included craniotomies for tumor (30), vascular malformation (6), epilepsy surgery (5), other lesions (3), rigid ventriculoscopy (9), and stereotactic biopsy (2). The accuracy of the system was measured by placing 20 fiducial markers on a plastic head of which 3-mm thickness CT images had been made. The average error using 4 fiducials as reference points was 1.5 mm when the fiducials were distributed around the circumference of the head. Using random surface points of 10, 20 and 40 in number gave average errors of 2.3, 1.6, 1.5 mm, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)