Several features which distinguish malignant melanoma arising in a Hutchinson's melanotic freckle (HMFM) from other types of malignant melanoma (MM) are described. Forty-eight patients with HMFM of the head and neck region were compared with 98 patients with MM of the head and neck region. All patients were clinical stage I. There was a preponderance of women amongst HMFM patients but not MM patients and HMFM patients were significantly older than MM patients. Although HMFM patients had thicker tumours than MM patients, these thicker lesions had a lower degree of mitotic activity and a higher incidence of partial regression. Overall prognosis for HMFM patients was significantly better than for MM patients, this being particularly so for women, none of whom died of melanoma. There was no close correlation between prognosis of HMFM patients and the thickness of their tumours. Every one of the HMFM in this study displayed evidence of severe solar degeneration, but such degeneration per se did not appear to confer upon these lesions their benign biological behaviour.