In order to assess the quality of life of patients with inoperable lung cancer, a questionnaire for patient self-administration with 29 variables was designed including the following subjects: psychosocial well-being, medical side-effects, activities of daily living and physical performance. The questionnaire was validated by using a semistructured interview. Thirty-one patients treated for inoperable lung cancer with either radiotherapy (42%), chemotherapy (42%) or a combination of these two treatment modalities (16%) were included in the study. The multitrait-multimethod matrix (MTMM) was used in the analysis of the construct validity. The present validation study showed a high degree of validity for the majority of the items studied. Some of the items showed a low degree of validity, and are subject to future analysis in large scale studies.