The evolution of more sophisticated imaging techniques has initiated a renewed interest in stereotactic devices, methods, and applications. The Brown-Roberts-Wells instrument was available to us early in its prototype stage, and this report reviews the first 500 cases using the system at the University of Southern California Medical Center Hospitals. Procedures were undertaken after recognition of apparent structural alterations on imaging studies, with objectives being both diagnostic and therapeutic. Target locations were predominantly within the cerebral centrum-basal ganglia (284 cases) and diencephalic-mesencephalic regions (129 cases). Operative objectives included: histological and microbiological assay, cyst and abscess aspiration, installation of temporary or permanent drainage conduits, point source and colloid base brachytherapy, cerebroscopy and ventriculoscopy with biopsy, aspiration, and excision, and intraoperative vascular localization. Using multiple instrumentation at the target point (741 point placements), we realized procedural objectives in 95.6% of the cases. The mortality was 0.2% and the morbidity was 1%: hematoma, 2 cases; infection, 1 case; increased deficit, 1 case; intraprocedural seizure, 1 case. A specific diagnosis was not obtained in 4.4% (necrosis, 10 cases; inflammatory response, 9 cases; granuloma, 1 case; gliosis, 1 case; diagnostic error, 1 case). Individual guidelines for case selection, technique, institutional requirements, and applications of the method are discussed.(ABSTRACT TRUNCATED AT 250 WORDS)