Adjuvant endocrine treatment-related adverse effects have a strong impact on patients' quality of life and thereby limit therapy's risk benefit ratio resulting in morbidity and treatment discontinuation. Still, many AI adverse effects remain untreated given that they are unrecognized by conservative methods (e.g., proxy ratings). The ability of complementary patient-reported outcomes (PROs) to provide a more comprehensive assessment of side-effects is to be explored. A cross-sectional study sample of 280 postmenopausal, early stage breast cancer patients was subjected to a comprehensive PRO assessment (FACT-B/+ES) at their after-care appointment. Prevalence and severity of patient-reported physical side-effects and psychosocial burden related to adjuvant AI therapy were compared with prevalences derived from pivotal phase IV trials (ATAC 2005, BIG1-98 2005). Across all symptom categories, highest prevalence rates were found for joint pain (59.6%), hot flushes (52%), lost interest in sexual intercourse (51.4%), and lack of energy (40.3%). Overall, PROs resulted in significantly higher prevalence rates as compared to physician ratings for all symptoms published in pivotal clinical trials except vaginal bleeding and nausea. The treatment duration exerted no significant impact on symptom frequency (P > 0.05). Established prevalence rates of endocrine treatment-related toxicity seem to be underestimated. The incorporation of PRO data should be mandatory or at least highly recommended in clinical treatment planning to arrive at a more accurate assessment of a patient's actual symptom burden enabling improved individualized management of side-effects and mediating the preservation of treatment adherence.