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The purpose of this study was the clinical and histo-immunohistochemical analysis of two cases: a cutaneous pigmented facial malignant melanoma and a lumbar congenital nevus with malignant transformation. A series of clinical elements raised the suspicion of some malignant melanocytic lesions and the histopathological analysis through the paraffin embedding technique confirmed the clinical suspicion. The immunohistochemical analysis using the streptavidin-biotin-peroxydase method of the facial malignant melanoma showed: S100 protein intense and diffuse positive, Tyrosinase diffuse positive, HMB45 strong and focal positive, Cyclin D1 positive in approximately 40% and Ki-67 positive in almost 70% of the tumor cells. The malignant melanoma developed on the nevocellular nevus displayed: S100 protein intense and diffuse positive, both in the nevus cells and in the malignant melanocytes as well, Tyrosinase intense and diffuse positive in the malignant melanocytes, poor and focal positive in the nevus cells and HMB45 intense and focal positive in the malignant cells and positive in the isolated nevus cells. Cyclin D1 was positive in about 70% of the malignant cells, but negative in the nevus area and Ki-67 was found positive in relatively 30% of the malignant melanocytes, also in less than 1% of the nevus cells. The pattern and the intensity of the Tyrosinase and HMB45 immunoexpression are important in the differentiation of the nevus cells from the malignant melanocytic cells. The immunoexpression of Cyclin D1 does not correlate directly with the proliferating activity of the malignant melanocytic cells in all types of malignant melanomas. [ABSTRACT FROM AUTHOR]