We describe 10 cases of primary well-differentiated neuroendocrine carcinomas (carcinoid tumor) of the lung with extensive sclerotic changes. The patients were 6 women and 4 men from 20 to 69 years of age. Clinically, patients had symptoms of bronchial obstruction such as cough, dyspnea, and chest pain. Surgical resection of the tumors was accomplished in all the cases. Histologically, all tumors corresponded to the well-differentiated type; however, in 4 cases, lymph node metastases were present. Immunohistochemically, all tumors showed positive staining for neuroendocrine markers, including chromogranin, synaptophysin, CD56, and broad-spectrum keratin. Follow-up information showed that 8 patients were alive after a period ranging from 1 to 5 years. The cases presented highlight an important feature of neuroendocrine carcinomas of the lung not previously addressed, one that may pose a problem not only in the diagnosis but also in the grading of these neoplasms.