Appropriate treatment for intracranial mass lesions depends upon accurate histological diagnosis. Although both advanced generation computed tomographic and magnetic resonance scanners can detect small lesions within the brain stem, only the combination of these advanced imaging tools with stereotactic instrumentation permits safe and accurate pathological diagnosis of such lesions. We present the results of 13 operations performed on 12 patients with mass lesions of the pons and mesencephalon. A definitive diagnosis was obtained in all patients. Aspiration of necrotic tumors (3 patients), neoplastic or benign cysts (2 patients), and chronic hematomas (2 patients) resulted in immediate neurological improvement in 7 of these 12 patients. No morbidity or mortality related to surgery occurred in this series. Both the preoperative clinical and radiographic diagnoses were erroneous in 6 patients so that accurate histological diagnosis indeed altered subsequent therapy. A transfrontal approach to the midbrain and a transcerebellar approach to the lateral pons are described. The importance of accurate diagnosis, the possibility of definitive therapy in selected patients, and the encouraging benefits and safety of stereotactic surgery indicate that empiric treatment of mass lesions of the midbrain and pons is no longer justified.