This retrospective study analyzed the outcome of lung cancer patients who were treated with either 4-MV or 10-MV photons. From October 1979 through December 1982, 126 patients with locally advanced, unresectable or medically inoperable, nonmetastatic non-small cell lung cancer were treated in a prospective trial in which they were randomly assigned to one of three chemotherapy combinations and thoracic radiotherapy. The patients were stratified by cell type, extent of operation, age, sex, and status of supraclavicular lymph nodes. All patients were followed until death or for a minimum of 4.8 years. Of the 102 evaluable patients, 98 were treated with either 4-MV or 10-MV photons (49 patients in each group). Outcomes examined included best primary tumor response, time to first local (in-field) recurrence, disease-free survival, and overall survival. No significant differences were detected between the patients treated with 4-MV or 10-MV photons for several important prognostic and treatment factors or for any of the study outcomes, including first local (in-field) recurrence, disease-free survival, and overall survival. For the group of 98 patients treated with either 4-MV or 10-MV photons, the estimated 2-year freedom from first local (in-field) recurrence was 47.7%. The estimated 2-year disease-free and overall survivals were 21.6% and 28.6%, respectively.