"Clark's nevi" is the name we apply to lesions that have been referred to in the past as dysplastic nevi or nevi with architectural and/or cytologic atypia. Our criteria for this histopathologic diagnosis include such architectural features as: (1) uneven distribution of melanocytes along the dermoepidermal junction; (2) irregularly spaced junctional nests that sometimes bridge between rete; and (3) ill-defined margins often characterized by a lentiginous growth pattern. If dermal nests are present, the junctional component usually extends laterally for some distance beyond the dermal nests. When there is cytologic atypia, it involves scattered melanocytes. Clark's nevi are of doubtful significance if few in number and occurring in a young patient in whom there is no family history of melanoma. When many are found in a patient with a family history of melanoma, their presence serves as a marker for dysplastic nevus syndrome (familial atypical mole-melanoma syndrome). When Clark's nevi develop in patients older than 40 or 50 years of age who have no family history of melanoma, their significance is less clear. However, they might signify a defect in those mechanisms that normally control formation and growth of melanocytic neoplasms.