Lung cancer is a major cause of death in many countries. To improve the results of treatment, more individualized therapy is necessary; for this, it is necessary to identify new prognostic factors. In 21 patients with lung cancer (17 with non-small-cell lung cancer and 4 with small-cell lung cancer) that had received radiation treatment, the amount of body protein was estimated with in vivo neutron activation analysis. Patients in whom body protein decreased had recurrences of the disease earlier and a poorer survival than patients whose body protein increased. A clear relationship was seen between changes in the body's protein content and recurrence-free survival. To better evaluate the prognostic value of body protein content in patients with lung cancer, a larger number of patients and a longer follow-up period are needed.