RESUMEN
We report a interesting case of acral lentigenous malignant melanoma associated with pseudocarcinomatous hyperplasia in a 65-year-old women A skin lesion showed a 5 x 5cm sized, black colored protruding mass with ulcerative surface on the right great toe for the last 10 years. Histopathological findings reveled hyperplasia of epidermis and the epithelium of the upper protion of the adnexa admixed with a proliferation of atypical epitheloid melanocytes that involved the mid and lower portion of the epidermis and the papillary dermis. There were dyskeratotic cells and pseudohorn cysts between the aggregates of atypical melanocytes. Immunhistochemical staining revealed that the nucleus and cytoplasm of the atypical epitheloid cells were strongly positive for S-100 protein and negative for keratin. Keratin staining revealed aggregates of positively stained kertinocytes with dyskeratosis and pseudohorn cysts admixed and trapped among negatively stained atypical melnocytes. A unusual case of malignant melanorna associated with pseudocarcinomatous hyperplasia in which conventional hematoxylin and eosin stained sections were difficult to differentiation between malignat melanoma and squamous cell carcinoma. Immunohistochemical staining for S-100 protein and keratin were essential in establesging the correct diagnosis.