AIM: To investigate observer variation in the diagnosis of thin cutaneous malignant melanoma and related lesions in a nationwide sample of histopathologists in the UK. METHODS: Out of a random sample of 195 pathologists, 148 (76%) participated in two circulations, the first with 20 slides and the second with 25 slides. The results were compared with those for the Cancer Research Campaign (CRC) Melanoma Pathology Panel, consisting of seven histopathologists and one dermatopathologist, which had developed and evaluated diagnostic criteria. RESULTS: In the first circulation, when no standardised diagnostic criteria were used, a fair level of agreement was achieved for an overall diagnosis using the categories benign naevi with no atypia, benign naevi with atypia and melanoma (Kappa = 0.45). This was low compared with the agreement of the panel who used agreed criteria (Kappa = 0.75). Moreover, participants in the nationwide survey were more likely to diagnose melanoma and less likely to diagnose benign naevi without atypia than the panel. In the second circulation, when diagnostic criteria and diagrams were used, there was a higher level of agreement for overall diagnosis using the categories benign, melanocytic intraepidermal neoplasia (MIN) with or without microinvasion and melanoma with vertical growth phase, and was the same as that achieved by the panel using the same criteria (Kappa = 0.68). CONCLUSIONS: As the incidence rate of thin melanomas has been increasing in the UK, it is important that standardised diagnostic criteria are used to ensure accurate reporting of incidence and correct management of patients. The use of MIN and the vertical growth phase seemed to be generally acceptable.