This epidemiological study was performed to evaluate the influence of postoperative complementary treatment with lectin-standardized mistletoe extract (sME) on breast cancer patients. The design (retrolective cohort analysis with parallel groups) and conduct of the study were in agreement with current standards for prospectively randomized clinical trials. A cohort of 1,248 breast cancer patients on postoperative chemo-, radio-, hormone-therapy were studied in 27 randomized centers. Patients with complementary medications other than sME were excluded from the evaluation and the final analysis was performed on data of 689 patients. From this cohort 219 patients received a complementary treatment exclusively with sME (therapy group), while 470 patients were without complementary treatment (control group). The median follow-up time was 284 days (therapy group) and 285 days (control group). The primary end-point of the study was to determine the impact of complementary sME treatment on disease- or therapy-induced adverse reactions in breast cancer patients. Imbalances for causal effects (covariates) were adjusted by propensity scores. Final evaluation was performed by estimating the linear regression between change in symptom score and propensity score with all data and using the regression line to calculate the change in symptom score expected for each patient. Tumor-associated events were evaluated by number and time until event. The safety of sME treatment was analysed in terms of number, severity, duration and outcome of adverse reactions. As compared to breast cancer patients without complementary treatment (control group), the administration of sME (therapy group) resulted in a significant reduction of adverse reactions induced by the tumor-destructive therapies (e.g. nausea, gastro-intestinal tract symptoms, depression, fatigue, mental symptoms) and prolonged relapse-free intervals, most pronounced for UICC stages IIa and IIb. The rate of sME-associated adverse reactions was 12.8%. All side-effects were mild to moderate, predominantly local skin reactions and self-limiting without therapeutic intervention. Complementary treatment of breast cancer patients with lectin-standardized mistletoe extract (sME) proved to be a well tolerated optimization of standard tumor-destructive therapies, mainly improving quality of life and relapse-free intervals in defined UICC stages.