The EORTC Quality of Life Study Group has developed a questionnaire for evaluating the quality of life of breast cancer patients, QLQ-BR23. This paper presents a psychometric study of a Spanish sample. 177 breast cancer patients who initiated their treatment have participated in the study. The QLQ-BR-23 questionnaire has been administered three times, before and during the treatment process, and in the follow-up period. Biographic and clinical data have also been recorded. A psychometric study of the reliability and validity of the questionnaire has been performed. Three scales have a Cronbach’s Alpha coefficient higher than 0.7 and two a lower one (0.53 and 0.62). Validity has been studied through one-way analysis of variance, in which several scales and item distinguish clearly between patients who differ in the clinical variables recorded, and also through repeated ANOVA measures, that reflect adequately the changes in the quality of life scores over time. These results indicate that the QLQ-BR-23 is a reliable and valid questionnaire when applied to a Spanish sample.