Based upon a review of cytologic specimens obtained from 46 patients, the morphologic characteristics of metastatic colon carcinoma and primary pulmonary adenocarcinoma were compared and contrasted. A monoclonal antibody (D-14) reported to be helpful in identifying colorectal carcinoma was also evaluated. Colon brushings and washings from 10 patients with colonic carcinoma and a variety of respiratory tract cytology samples from 16 and 20 patients with metastatic colonic and primary pulmonary adenocarcinomas, respectively, formed the basis of this review. The presence of well-formed glands and dirty necrosis was significantly more characteristic of colon carcinoma. Other cytologic features and D-14 staining were not sufficiently distinctive to allow a separation from primary pulmonary adenocarcinoma.