Anorectal melanoma is a rare disease with an exceedingly poor prognosis. Literature evaluating outcomes is often difficult because of the infrequent diagnosis, delayed presentation and almost universally poor outcome. We describe a case of a 78-year-old woman with a large anorectal melanoma without detectable metastasis following radiological investigation. Due to the patient's co-morbidity, radiotherapy of 36 Gy once weekly over 7 weeks, was given as the sole treatment. The clinical outcome showed a remarkable regression of the tumour without radiological or clinical recurrence at 12 months' follow-up. This is the first case of the successful use of radiotherapy alone in anorectal melanoma.