Summary Objectives: The aim of this study was to assess validity and reliability of the EORTC QLQ C30 and BR23 questionnaires among patients with breast neoplasm in Poland. Material and methods: Analysis was done on the basis of research conducted at the First Department of General Surgery, JUMC, Krakow, among 105 women. Two aspects of questionnaire validity have been analyzed: construct validity using principal component analysis and criterion validity evaluated by comparison of two clinical subgroups of patients. Reliability analysis was done by estimating internal consistency using Cronbach’s alpha coefficient and correlations between items and hypothesized scales. In addition, test-retest reliability was evaluated by calculating the correlation coefficient between two assessments, done 6 and 12 months after treatment. Results: All scales met Nunnally’s criterion except for the scale of nausea and vomiting. Almost all variables met Kline’s criterion, except for two variables: one from the scale of physical functioning and the other a part of the scale of systemic therapy side effects. High correlations obtained by the test-retest method confirmed the reliability of all scales. Analysis showed some fault in the theoretical validity of the scale of systemic therapy side effects. Nonetheless, criterion validity was confirmed for all scales. Conclusions: Results have confirmed the validity and reliability of the EORTC QLQ C30 and BR23 tests as instruments for quality of life evaluation in Polish patients.