Background: The European Organization for research and Treatment of Cancer (EORTC) Core Quality of Life Questionnaire (QLQ-C30) scales are scored on a 4-point response scale, ranging from not at all to very much. Previous studies have shown that the German translation of the response option quite a bit as massig violates interval scale assumptions, and that ziemlich is a more appropriate translation. The present studies investigated differences between the two questionnaire versions. Methods: The first study employed a balanced cross-over design and included 450 patients with different types of cancer from three German-speaking countries. The second study was a representative survey in Germany including 2033 respondents. The main analyses included compared the ziemlich and massig version of the questionnaire using analyses of covariance adjusted for sex, age, and health burden. Results: In accordance with our hypothesis, the adjusted summary score was lower in the massig than in the ziemlich version; Study 1: - 4.5 (95% CI - 7.8 to - 1.3), p = 0.006, Study 2: - 3.1 (95% CI - 4.6 to - 1.5), p < 0.001. In both studies, this effect was pronounced in respondents with a higher health burden; Study 1: - 6.8 (95% CI - 12.2 to - 1.4), p = 0.013; Study 2: - 4.5 (95% CI - 7.3 to - 1.7), p = 0.002. Conclusions: We found subtle but consistent differences between the two questionnaire versions. We recommend to use the optimized response option for the EORTC QLQ-C30 as well as for all other German modules.