Carcinoid tumorlets are relatively common, generally incidental, pathologic findings. Although not typically identified grossly, carcinoid tumorlets have been described as gray-tan nodules measuring millimeters in size; 5 mm has been considered an arbitrary size for which carcinoid nodules should be termed carcinoid tumors. Carcinoid tumorlets may be seen in association with carcinoid tumors and/or neuroendocrine-cell hyperplasia, and some schemes propose an evolution from neuroendocrine-cell hyperplasia to tumorlet to carcinoid tumor. Differential diagnosis includes typical carcinoid, neuroendocrine-cell hyperplasia, minute pulmonary meningothelial-like nodules (chemodectomas), small-cell carcinoma, and lymphangitic metastases (see Chapter 10, Part 6 and Chapter 14, Part 4).