To evaluate the primary site, pulmonary hilar, and mediastinal lesions in patients with lung cancer, thallium-201 using single-photon emission computed tomography (SPECT) was used as a tumor agent. The study population consisted of 7 patients (5 men and 2 women), aged 57 to 82 years (mean 68). Tl-201 tomography imaged positive at the primary site in all patients with lung cancer. The images demonstrated the tumor and/or mediastinal lymph nodes that were greater than 2.0cm. In particular, each tomograms facilitated the identification of the Tl-201 increased uptake in proportion to primary site. The short waiting period after injection with Tl-201 clearly offers a major advantage over alternative tumor imaging agents, such as Ga-67 or radiolabeled monoclonal antibodies. Furthermore, Tl-201 SPECT may be useful to detect the tumor and/or mediastinal lymph nodes in patients with lung cancer.