51 patients with medically refractory Parkinson's disease underwent stereotactic posteromedial pallidotomy between August 1993 and February 1997 for treatment of bradykinesia, rigidity, and L-DOPA-induced dyskinesias. In 29 patients, the pallidotomies were performed with the Leksell Gamma Knife and in 22 they were performed with the standard radiofrequency (RF) method. Clinical assessment as well as blinded ratings of Unified Parkinson's Disease Rating Scale (UPDRS) scores were carried out pre- and postoperatively. Mean follow-up time is 20.6 months (range 6-48) and all except 4 patients have been followed more than one year. 85 percent of patients with dyskinesias were relieved of symptoms, regardless of whether the pallidotomies were performed with the Gamma Knife or radiofrequency methods. About 2/3 of the patients in both Gamma Knife and radiofrequency groups showed improvements in bradykinesia and rigidity, although when considered as a group neither the Gamma Knife nor the radiofrequency group showed statistically significant improvements in UPDRS scores. One patient in the Gamma Knife group (3.4%) developed a homonymous hemianopsia 9 months following treatment and 5 patients (27.7%) in the radiofrequency group became transiently confused postoperatively. No other complications were seen. Gamma Knife pallidotomy is as effective as radiofrequency pallidotomy in controlling certain of the symptoms of Parkinson's disease. It may be the only practical technique available in certain patients, such as those who take anticoagulants, have bleeding diatheses or serious systemic medical illnesses. It is a viable option for other patients as well.