The occurrence of nuclear crush artifact (NCA) in cytologic specimens of small cell undifferentiated carcinoma (SCUC) and other carcinomas of the lung, lymphoma and benign lymphoid proliferations was studied to determine its diagnostic usefulness. NCA was found to be a common and morphologically distinct feature in SCUC, depending on how the specimen was prepared; it was present in aspirates, washings, brushings and sputum, but not in pleural or cerebrospinal fluids. It was absent in other types of cancer and could be distinguished from other similar-appearing artifacts. These results demonstrate that NCA is a diagnostically useful feature in pulmonary SCUC.