Seventy-five patients with lung cancer underwent a gallium scan and thoracotomy with total mediastinal nodal dissection. Evaluation of mediastinal lymph nodes by means of the gallium scan showed a sensitivity of 23 percent (3/13), a specificity of 82 percent (31/38), an accuracy of 67 percent (34/51), a positive predictive valve of 30 percent (3/10), and a negative predictive value of 76 percent (31/41) in those patients whose primary tumors demonstrated uptake of radioactive gallium. The low sensitivity was due to an inability to detect microscopic disease in mediastinal lymph nodes. The specificity was decreased by gallium-67 uptake in enlarged inflamed nodes that contained no metastases. These results do not support the use of the gallium scan in the selection of patients with lung cancer for thoracotomy.