The term "nevoid malignant melanoma" (nevoid MM) is used here to describe rare nodular malignant melanomas that may escape detection in routine histological sections due to the lack of a prominent intraepidermal component, sharp lateral circumscription and evidence of partial maturation with descent in the dermis. Nevoid MM mimic ordinary compound or intradermal melanocytic nevi when the melanoma cells are small, or Spitz's nevi when the cells are large. The patterns of HMB-45 staining in 12 nevoid MM were compared with those in 107 melanocytic nevi. HMB-45 staining was strong in the dermal component of the nevoid MM, even in the absence of a junctional component. In common acquired and congenital nevi, the upper dermal component stained less than the junctional component of the lesion. The deepest components of these nevi were negative. Spitz nevi and cellular blue nevi had positive dermal cells, even without a junctional component. Additional staining for a proliferation marker, such as cyclin (PCNA) or Ki-67 (with the antibody MIB-1), can help further in distinguishing a nevoid MM from a Spitz's nevus. Melanoma has strong nuclear staining throughout the lesion. In contrast, Spitz's nevi have more staining at the top of the lesion than at the bottom. The patterns of HMB-45 and MIB-1 staining can be used along with standard histologic criteria for the diagnosis of nevoid MM. Clinicopathologic correlation is needed to distinguish some metastatic melanomas from primary nevoid MM.