A retrospective study of 294 patients with biopsy- or autopsy-proven adenocarcinoma of the pancreas was done. The initial diagnosis of ten patients (3.4%) was primary lung cancer. All ten patients were cigarette smokers. Hilar adenopathy with mediastinal widening was the most common roentgenographic appearance. Tumors of the body and tail of the pancreas more often appeared to be primary lung tumors than did tumors of the head of the pancreas (about 10% vs. about 1%). Adverse consequences of this unrecognized phenomenon may include unnecessary lung surgery for some cancer patients and overreporting of deaths from lung cancer.