In a consecutive series of 62 lung resections for bronchogenic adenocarcinoma, 12 patients (19 percent) were found to have two or more adenocarcinomas on careful pathologic examination. These tumors all met the criteria for separate primary malignancy. In only two of the patients were the additional lesions suspected preoperatively. This incidence of multiple primary lung adenocarcinomas in apparently operable patients is several fold higher than would be anticipated from the literature. The phenomenon has important implications for preoperative radiologic evaluation, postoperative pathologic examination, assignment of TNM stage, and clinical follow-up of patients undergoing successful resection.