Background Classical cyclophosphamide, methotrexate, 5-fluorouracil (CMF) including oral cyclophosphamide is still considered an important adjuvant chemotherapy regimen in patients with early breast cancer (BC). Concern has been raised regarding the feasibility of this regimen, especially in post-menopausal patients. Patients and methods 254 pre- and post-menopausal node-positive BC patients aged ≤70 years received six cycles of CMF in the context of a Belgian multicentric phase III trial of adjuvant chemotherapy. CMF dose and schedule were as follows: cyclophosphamide 100 mg/m2 p.o. on days 1 to 14, methotrexate 40 mg/m2 intravenously (i.v.) on days 1 and 8, 5-fluorouracil 600 mg/m2 i.v. on days 1 and 8; cycles q. 28 days. The relative dose intensity (RDI) was calculated as the ratio between the delivered DI and the planned DI. We also analysed the RDI in two subgroups of patients with age ≥50 years or <50 years. Results Overall, the percentage of patients ending the six cycles of the planned CMF regimen was 90%. The mean RDI achieved in the population of 254 patients was 90% (range 8% to 129%). The subgroup analysis of patients aged ≥50 years and <50 years showed that 81% and 76% of patients, respectively, received ≥80% of the planned chemotherapy dose intensity ( P = 0.33). No statistically significant difference was found between the percentage of patients who received a RDI <80% and the participating institutions ( P = 0.50). Conclusions The classical CMF regimen was a feasible regimen in the context of a multicentric trial, in which academic institutions as well as community hospitals participated. No substantial differences in RDI and cumulative doses were found in relation to a patient’s age and the participating institution.