Patients with breast cancer receiving adjuvant chemotherapy frequently suffer from a restricted quality of life (QoL) due to the side-effects of chemotherapy and the consequences of coping with the diagnosis. Therefore, the objective of this clinical study was to investigate the impact of PS76A2, an aqueous mistletoe extract standardised to the galactoside-specific mistletoe lectin, on QoL by performing a placebo-controlled trial. Overall, 272 patients with breast cancer receiving adjuvant CMF chemotherapy (cyclophosphamide-methotrexate-fluorouracil) were enrolled and randomised to groups receiving placebo or PS76A2 at concentrations of 10, 30 or 70 ng mistletoe lectin (ML) per ml. The patients received 0.5 ml study medication twice weekly subcutaneously for 15 consecutive weeks (4 CMF cycles). Primary variables were the self-assessment QoL scores GLQ-8 (Global Life Quality) and Spitzer's uniscale. As a result, statistically significant effects on QoL were obtained with the medium dose (15 ng ML/0.5 ml). The treatment difference between the medium dose and placebo with regard to the GLQ-8 sum was 60.8 mm (95% confidence interval: 19.3 to 102.0 mm). The treatment effect for Spitzer's uniscale between the medium dose and placebo was 16.4 mm (95% confidence interval: 6.3 to 26.6 mm). The results on QoL were supported by an increase of T helper lymphocytes (CD4+) and the CD4+/CD8+ ratio (p<0.05). Overall, PS76A2 was well tolerated. Local reactions at the injection sites occurred dose-dependently, but were mild at the low and medium dose levels.