The European Organisation for Research and Treatment of Cancer Quality of Life Core Questionnaire (EORTC QLQ-C30) was evaluated for its psychometric properties in a sample of cancer patients from the culturally distinct South Asian subcontinent, which accounts for a significant proportion of the global cancer burden.Psychometric testing assessed the hypothesised scale structure, internal consistency, content and construct validity, and acceptability of the Sinhala version of the QLQ-C30 independently in two heterogeneous groups of cancer patients at pretreatment (N = 489) and during treatment (N = 343).Qualitative feedback from an expert panel assessing content validity recommended measuring family support during illness as an additional, culturally-relevant dimension of health-related quality of life (HRQL). Compliance was high (100%), with little missing data (0.11%). Multitrait scaling results supported the scale structure of the QLQ-C30, with the exception of the cognitive functioning scale, which was also the only scale that did not meet the 0.70 internal consistency criteria in either sample. Interscale correlations were of a moderate size, with conceptually related scales showing higher correlations. All scales were able to discriminate clearly between pre- and current treatment patients (P < 0.01), although results were less consistent when comparing groups formed on the basis of age and disease stage.Overall psychometric results confirmed the QLQ-C30 as a reliable and valid questionnaire for assessing HRQL of cancer patients in Sri Lanka.