Sections from 95 skin lesions excised at pigmented lesion clinics in England and Scotland were studied by eight histopathologists in order to evaluate consistency in the use of histopathological terms for features of diagnostic and prognostic importance for cutaneous malignant melanoma. The level of agreement (kappa) amongst the panel improved after discussion and re-definement of criteria for several features. These included, architectural and nuclear atypia, pagetoid infiltration and radial and vertical growth phases. A high level of agreement was achieved for an overall benign or malignant diagnosis (kappa = 0.77) but use of more specific terms such as benign naevi with atypia and melanoma < or = 0.76 mm thickness, was associated with only an intermediate level of agreement. Of the original diagnosis of melanoma, 17% were re-classified by the panel as benign with atypia and 2% reported to be benign were judged to be melanoma. This reflected the high proportion of borderline lesions in the study. The use of standardized diagnostic criteria with precise definitions has been shown to improve consistency in diagnosis and it is recommended for general application. From this should emanate more reliable incidence figures for thin melanoma, and improved understanding of the nature of these early lesions, to the benefit of patient and clinician alike. The poor concordance in distinguishing severe dysplasia in the junctional component of melanocyte proliferations from melanoma in situ and superficial dermal invasion improved only modestly despite intensive efforts. Since melanoma in situ and severe dysplasia cannot be distinguished by objective measurements and since their clinical management is the same, the panel suggests that attempts to separate them in diagnostic reports should be discontinued and they could both be referred to as melanocytic intraepidermal neoplasia (MIN). If it becomes accepted that dermal invasion without a vertical growth component can also be managed identically to MIN, then this invasive radial phase may be appropriately referred to as microinvasion and linked to MIN for the purposes of clinical management.