BACKGROUND: English and Chinese are two of the most widely used primary languages in the world. Patients in many cancer centers have a variety of ethnic backgrounds and primary languages. The comparability of version 4 of the English and Chinese versions of the Functional Assessment of Cancer Therapy-General (FACT-G) and version 3 of the European Organization for the Research and Treatment of Cancer Core Quality of Life Questionnaire (EORTC QLQ-C30) have not been established. METHODS: In total, 1136 ethnic Chinese patients with cancer were recruited from the National Cancer Centre of Singapore. Patients chose to answer an English or Chinese questionnaire, according to their own preference. Multiple regression analysis was used to adjust for differences in demographic and health characteristics. Equivalence was confirmed if the 90% confidence intervals of the adjusted mean difference fell completely within an equivalence zone of +/- 0.25 standard deviations (SD). RESULTS: The English and Chinese versions of the Total, Emotional, and Functional Well Being Scales of the FACT-G and the Physical and Emotional Functioning Scales of the EORTC QLQ-C30 were equivalent. Scores for the other scales on the two questionnaires, at most, had a small differences that did not exceed 0.5 SD. Nevertheless, the Chinese translation of the question "I have a lack of energy" in the Physical Well Being Scale of the FACT-G produced results that differed from the results produced by the original English version. CONCLUSIONS: Data collected from English-speaking and Chinese-speaking respondents were capable of being pooled, and either version could be used for bilingual respondents. Nevertheless, the authors recommend modification of the Physical Well Being question that produced different results ("I have a lack of energy").