The authors report on the results of the immune monitoring of a study population of 31 patients with lung cancer who were treated with radiotherapy. A synthetic thymic pentapeptide, thymopentin, was employed whose effect was evaluated on the immunological parameters analyzed. After radiotherapy, a considerable and homogeneous decrement was observed in several lymphocytic subsets (less sensible in activated T-cells), together with a progressive decrement in the helper/suppressor ratio, in the long run. Monocytes and null cells showed more radioresistance. Thymopentin had no influence on the tested immunological parameters up to 6 months after radiotherapy; later on, a slightly more balanced helper/suppressor ratio could be noticed in the surviving patients who had been treated with thymopentin.