Dysplastic nevi are an important indicator of risk of cutaneous malignant melanoma. The study of and, particularly, international communication regarding this group of lesions have been hindered by a lack of precision in diagnosis. In an effort to broaden understanding, a panel of pathologists agreed upon a set of criteria for the diagnosis of dysplastic melanocytic nevi. Two major and four minor criteria were defined. The major criteria are (1) basilar proliferation of atypical nevomelanocytes (extending at least three rete ridges or "pegs" beyond any dermal nevo-cellular component), and (2) organization of this proliferation in a lentiginous or epithelioid-cell pattern. Minor criteria are (1) the presence of lamellar fibrosis or concentric eosinophilic fibrosis, (2) neovascularization, (3) inflammatory response, and (4) fusion of rete ridges. Diagnosis required presence of both major criteria and at least two minor criteria. One hundred fourteen histologic specimens of benign acquired nevi, dysplastic nevi, and radial-growth-phase melanomas were examined by the members of this panel; their diagnoses were compared to determine degree of concordance. The established criteria yielded 92% mean concordance overall.