Thalamotomy aiming at the CM-Pf complex and using stereotactic gamma irradiation has been performed in a series of 52 patients with severe pain due to malignancy. Lesions were produced either contra- or ipsilaterally to the side of the pain as well as bilaterally. Eight patients experienced good pain relief, 18 had moderate relief, and in 24 the operation did not significantly influence the pain. A second operation following recurrence of pain was rarely of value. There was a tendency towards more efficient relief of pain located in the face or in the arm and shoulder than of pain in the lower part of the body. Although contralateral lesions seem to be most effective, ipsilateral operations may also give some relief. The best results were obtained when the lesions were placed close to the wall of the third ventricle and at the level of the posterior commissure. Postmortem examination of 21 brains disclosed that the mean error in the placing of the lesions was about 1 mm. It is concluded that medial thalamotomy may be tried as a last resort in the treatment of cancer pain in selected patients with a short life expectancy.