PURPOSE: The aim of this study was to evaluate the multidimensional constructs of the EORTC Quality of Life Questionnaire (QLQ-C30) in patients with cancer, employing not only the commonly used multitrait scaling analysis and interscale correlations, but also the factorial and multidimensional scaling (MDS) analyses. MATERIALS AND METHODS: A total of 334 Korean cancer patients participated in this cross-sectional study. All patients completed the QLQ-C30. RESULTS: With the multitrait scaling analysis, the cognitive functioning scale did not meet item convergent and divergent validities. With the interscale correlations, the physical and role functioning scales were found to be highly correlated; this was also evident in the factorial analysis. The MDS showed that each item within the social, emotional, global health status/quality of life, and nausea/ vomiting scales were clustered close together, but far from those of the other scales. CONCLUSION: The authors conclude that the four way evaluation of the QLQ-C30 produced results that supported the original hypothesized constructs. However, the physical and role functioning scales were not distinctive, and that of the cognitive functioning was somewhat problematic in the Korean population with cancer.