With the increasing use of breast-conserving therapy plus systemic chemotherapy for the treatment of early breast cancer, the optimal sequencing of radiation therapy and chemotherapy remains controversial. Sequencing of therapy may influence not only treatment delivery, but control rates, complications and cosmesis. The aim of this study was to evaluate whether concurrent sequencing of standard doses of CMF (cyclophosphamide, methotrexate and 5-fluorouracil) and adjuvant radiation therapy for early breast cancer impacted on optimum treatment delivery. As both an intravenous (i.v.) 3-week regimen and classic (standard) CMF were utilised in this study, both types of CMF were compared. The effect of sequencing on complications and treatment delays were also assessed. 116 patients treated with CMF chemotherapy and adjuvant tangent breast radiation were studied. 73 patients were treated prospectively with concurrent therapy and were retrospectively compared with a matched group of 40 patients treated with sequential or sandwich therapy. All patients had stage 1 or 2 cancers. There were no planned dose reductions introduced for either treatment modality. Concurrent sequencing had no impact on the ability to deliver optimum radiation or chemotherapy doses. There was no significant difference in acute Radiation Therapy Oncology Group (RTOG) skin reactions or complications between the two groups. Although small, there was a significant delay (1.32 days (0-15 versus 0.36 (0-7)) in the concurrent group (P=0.03) in the delivery of radiation therapy. Sequencing had no significant effect on haematological parameters. 'Standard' CMF had a more profound effect on treatment delivery than i.v. CMF (Radiation delay 2.2 days versus 0.26, P=0.002, % chemotherapy delivered 93% versus 99% P=0.000004). At a mean follow-up of 2.6 years, there was no difference in the cosmetic scores between the two groups. Both local and distant control rates were excellent. This study has shown that standard radiation therapy can be delivered safely concurrently with CMF chemotherapy. Whether this approach may lead to better control rates in the future needs further study.