The Turkish version of the European Organization for Research and Treatment of Cancer Quality of Life Core Questionnaire version 2.0 (EORTC QLQ-C30 v.2.0) has started to be used in clinical trials recently. The objective of the study was to evaluate the validity and reliability of the Turkish version of the EORTC QLQ-C30 v.2.0 and the correlation between the Karnofsky Performance Scale (KPS) and the EORTC QLQ-C30. Two hundred and two lung cancer patients were included in the study between January and March 2000. All the subscales met the minimal standards of reliability (Cronbach's alpha greater than or equal to 0.70). Only the role functioning scale differed among the three disease stages of patients (local, locoregional and metastatic). There was no statistically significant difference among therapy types. All interscale correlations were statistically significant (P < 0.01). The strongest correlations were found among the physical functioning, role functioning and fatigue scales. Social functioning was closely related with physical, role, emotional and cognitive functioning. The weakest correlations were between nausea/vomiting and the other scales. Global quality of life (QOL) was substantially correlated with most of the scales except cognitive functioning. The coefficients for the correlation between the items differed between 0.12 and 0.97 and all the subscales were strongly correlated with the scales which they formed. The highest correlation between the EORTC QLQ-C30 and KPS was for physical functioning (r = 0.62, P < 0.05). The Turkish version of the EORTC QLQ-C30 is a valid (by means of interscales validity) and reliable instrument for Turkish lung cancer patients and can be used in clinical studies but needs supporting by the reference data on the QOL of the Turkish population.