AbstractIn five patients with malignant mesothelioma of pleura, malignant cells, presumably derived from intrapulmonary deposits, were found in sputum specimens. The cells presented mainly as papillary aggregates of epithelial‐like cells, in most cases without specific identifiable mesothelial cell features. In one patient, the specimen was received early in the diagnostic work‐up and led to a strong consideration of the diagnosis of adenocarcinoma. In the remainder, the finding was incidental, occurring after the diagnosis of mesothelioma had been established. The cases are reported to draw attention to this phenomenon, previously unreported in the cytologic literature, and to emphasize that the finding of malignant cells in sputum does not preclude the diagnosis of malignant mesothelioma. Diagn Cytopathol 1986;2:21