The importance of obtaining an intraoperative diagnosis and measurement of thickness of a malignant melanoma has emerged since the change in the surgical approach towards thin melanomas (0.75 mm). In experienced centers, both diagnosis and thickness, can be established on frozen section. Eighty-four pigmented lesions were examined on frozen section. Thirty of 31 were correctly diagnosed as malignant melanomas. The one not diagnosed was a regressing melanoma. The frozen section diagnosis of a regressing melanoma is difficult, and this should be deferred until paraffin sections are examined. In 29 consecutive skin melanomas thickness was measured on frozen section and was found to be 0.1-0.4 mm more than that measured on paraffin sections of the same specimens. It is therefore suggested that tumors measuring up to 0.85 mm on frozen section should be included in the group of thin melanoma.