Between 1981 and 1991, seventy-two cases of brain stem lesions have undergone CT or MRI-directed stereotactic surgery using the Brown-Roberts-Wells or Cosman-Roberts-Wells stereotactic systems. In 16 cases, the pre-operative diagnosis of haematoma had been made and the procedure was for therapeutic aspiration. In fifty-six cases, the pre-operative diagnosis was uncertain although intrinsic tumour was suspected. Histological diagnosis was obtained in fifty-four of these and unexpected findings of haematomas, granulomas, leucoencephalopathy, demyelinating disease, vasculitis and radiation necrosis occurred in over 15% of the cases. There were no operative deaths and morbidity was low. Advances in neuroradiological imaging with CT and MRI have increased the detection rate of brain stem lesions and image-directed stereotactic biopsy makes it possible in most cases to obtain histological diagnosis with relatively low morbidity which is important for treatment and prognosis. MRI-directed biopsies can compliment CT-guided biopsies in most cases where the lesion is not adequately delineated on the CT image.