PURPOSE: To investigate whether whole brain irradiation might be replaced by focalized irradiation after resection of a single brain metastasis in patients where extracranial tumor control is deemed to be obtained. PATIENTS AND METHODS: Twelve patients were introduced in a phase I/II prospective study of conformal postoperative external irradiation after resection of a solitary brain metastasis. The radiation treatment consisted of 50.4 Gy (1.8 Gy per fraction, five fractions per week). The planning target volume consisted of the tumor bed and a 2 cm safety margin. All treatments were optimized with head immobilization, dedicated tomodensitometry and computer assisted three-dimensional treatment planning. RESULTS: The median survival was 7.2 months (range 2.4-50.4 months). Eleven of the 12 patients died. Eight of the 12 patients presented intracranial recurrence and seven died as a consequence of intracranial tumor progression. CONCLUSIONS: Focalized external irradiation cannot serve as a reasonable alternative to whole brain radiotherapy (WBRT) even for patients with apparently one single resected brain metastasis. The dogma of 'one metastasis = multiple metastases' seems to be confirmed.