Forty one cases of large cell anaplastic carcinoma of the lung (LCACL) were investigated by electron microscopy and immunoperoxidase studies for cytokeratin, enolase, and carcinoembryonic antigen. The results indicated that these neoplasias, grouped as an unique entity by ordinary histopathologic findings, may be further divided into five groups as follows: squamous, adenomatous, adenosquamous, neuroendocrine, and undifferentiated. The authors suggest that this subclassification may be useful in treatment orientation and in the prognostic evaluation of these neoplasias.