Adenosquamous carcinoma of the lung is a rare and poorly described entity. At the University of Chicago between 1974 and 1985, 2.3% (20/873) of patients with lung cancer had well-differentiated adenosquamous carcinoma. As in non-small cell lung cancer, patients with Stage I disease were amenable to operation with 60% (3/5) free from disease between one and six years postoperatively. However, Stage II adenosquamous carcinoma (14 patients) exhibited highly aggressive behavior with rapid progression of disease (mean interval, 2.1 months). Despite combinations of surgery (6 patients), chemotherapy (6 patients, one response), and radiotherapy (10 patients, no response), median survival for patients with Stage III adenosquamous carcinoma was 5.0 months, worse than that for Stage III small cell cancer (9.6 months), adenocarcinoma (9.0 months), and squamous cancer (7.8 months).