Under a ruling by the Bundesgerichtshof from December 17, 1991 (AZ VI ZR 40/91), the surgeon is obliged to inform a patient about the risks of a possible blood transfusion if the transfusion frequency is 5% or more. Between January 1989 and December 1994, 273 patients with primary breast cancer at the stages pT1-3, N1-2, M0 underwent one-time modified radical mastectomy (n = 164) or a breast conserving operation (n = 109) in our hospital. The number of blood transfusions, pre- and postoperative haemoglobin, age, tumour size, lymph node involvement, kind and duration of the operation and the postoperative course were analysed. In all, 44 of the 273 patients (16.1%) received an homologous blood transfusion perioperatively. The annual transfusion rates were 39.4% (1989), 44.8% (1990), 12.2% (1991), 16.6% (1992) and 3.1% (1993), falling to 2.2% in 1994. The mean pre- and postoperative haemoglobin concentrations did not differ significantly (p > 0.05) over the years: 13.4 +/- 1.0 g/dl and 12.0 +/- 1.1 g/dl in 1989 and 13.7 +/- 1.2 g/dl and 11.8 +/- 1.2 g/dl in 1994. Patients who received transfusions had preoperative anaemia (p < 0.021), bigger tumours (p < 0.0005) and mastectomy operations significantly more frequently than patients not given transfusions. There were no correlations between transfusions and age, lymph node involvement and kind and duration of operation. We conclude that, cognizant of a transfusion frequency of 2.2% in our hospital in one-time breast cancer operations, only patients with anaemia or large tumours require blood transfusions, for which autologous blood donations or normovolaemic haemodilution are the choices. The patients' attention is to be drawn to these. In cases of normal preoperative haemoglobin and small tumours, the physician should inform the patient that experience has shown that in all probability a blood transfusion will not be necessary and so a preoperative autologous blood donation or normovolaemic haemodilution can be dispensed with.