A man aged 26 years had been subject to frequent transient episodes of left homonymous hemianopia, initially accompanied by visual hallucinations and left miosis, since the age of 8 years. These partial seizures recurred 3 to 30 times each month in spite of treatment with anticonvulsant medication. The causative lesion, which had escaped detection by cerebral angiography and repeated computed tomographic scanning, was shown by magnetic resonance imaging to be a small cavernous hemangioma in the right occipital lobe. This was treated by a course of fine-beam radiotherapy directed to the lesion. After treatment the partial attacks ceased, although one nocturnal clonic seizure occurred 12 months after radiotherapy and 6 weeks after stopping all anticonvulsant medication.