OBJECTIVES: The purpose of this investigation was firstly to assess the overall frequency of subjectively experienced symptoms self-reported by patients receiving endocrine therapy and secondly to compare these symptoms with side effects assessed by clinicians in pivotal trials. METHODS: Unselected patients with early and advanced breast cancer receiving endocrine therapy were approached consecutively during a routine outpatient visit. They received a questionnaire called Checklist for Patients with Endocrine Therapy (C-PET), a validated self-assessment tool to determine prespecified symptoms associated with endocrine therapy. Data on toxicity were also obtained from previously published trials. RESULTS: 405 patients were approached and 373 agreed to participate in this study. Some symptoms were significantly more often recorded by the women in the adjuvant setting completing the C-PET than by physicians' reports in pivotal trials: hot flushes/sweats (C-PET 70%, ATAC 40% and BIG1-98 38%), low energy (C-PET 45%, ATAC 15% and BIG1-98 9%), fluid retention (C-PET 22% and BIG1-98 7%) and vaginal dryness (C-PET 30% and BIG1-98 3%). Similar differences were observed in the metastatic and adjuvant setting. CONCLUSIONS: A simple tool like the C-PET questionnaire is able to reflect the treatment burden of endocrine therapies and may be helpful to improve communication between patients and care providers. Some symptoms were significantly more often reported by the women in the C-PET than by physicians in pivotal trials.