True histiocytic neoplasm of Langerhans' cell type.

Malignant histiocytic neoplasms of dendritic cell lineage are rare and most are derived from the dendritic cells of the T-cell and B-cell areas of the lymph node, respectively. Only one case of a malignant histiocytic neoplasm of Langerhans' cells, the dendritic cell of the skin, has been reported. We report the occurrence of a true histiocytic neoplasm of Langerhans' cell type in a 23-year-old female patient. The diagnosis of this highly aggressive neoplasm is supported by histochemical, immunohistochemical, ultrastructural, and genotypic analysis--techniques which allow differentiation from malignant melanoma and large cell anaplastic lymphoma.