The concurrent administration of adjuvant chemotherapy and radiotherapy in breast cancer treatment might lead to an increased incidence of side-effects. In this prospective, non-randomised, comparative study, the acute toxicity of radiotherapy alone (RT) and radiotherapy concurrent with doxorubicin-cyclophosphamide (AC/RT) and radiotherapy concurrent with cyclophosphamide-methotrexate-5-fluorouracil (CMF/RT) was compared. We used the common toxicity criteria (CTC) to score the level of acute toxicity before, during and 6 months after the completion of the period of irradiation. The number of hospital admissions, as well as the compliance of chemotherapy, were noted. We observed that patients treated with AC/RT and CMF/RT had significant higher incidences of (high-grade) skin-toxicity, oesophagitis, dyspnoea, malaise, anorexia, nausea and hospital admission compared with those treated with RT only. The target-volume of radiotherapy was the main predictor of (high-grade) acute skin toxicity and oesophagitis. AC/RT was associated with significant more (high-grade) skin toxicity than CMF/RT. The dose of chemotherapy was reduced to less than 85% of the planned dose in 11% of patients, 17% of patients treated with concurrent chemotherapy and radiotherapy needed admission to hospital. From the results of our study, we conclude that the concurrent administration of adjuvant chemotherapy and radiotherapy leads to an unacceptably high level of acute toxicity.