Two series of cutaneous malignant melanoma (CMM), one from a low incidence area (the Oxford Region) and one from a high incidence area (Western Australia), were compared by one pathologist using the same histological criteria. The findings included similarities in the distributions of histogenetic type of CMM, predominant cell type, and the degree and pattern of inflammatory reaction. Differences were found in other features including tumour profile, the presence of ulceration, mitotic activity, evidence of regression and level of invasion. The most important, and consistent morphological difference was the greater thickness of tumours from the Oxford Region, which is thought to be consistent with the theory that CMM is diagnosed at an earlier stage in high incidence areas because of greater medical and public awareness of the condition. Some problems in the use of accepted classifications of CMM are discussed.