Acute toxicity has been evaluated in head and neck cancer patients treated with intensity-modulated radiotherapy using simultaneous integrated boost (SIB-IMRT). The basis of the treatment protocol is an irradiation in 30 fractions with a total dose: 66 Gy to the region of macroscopic tumor, 60 Gy to the region of high-risk subclinical disease and 54 Gy to the region of low-risk subclinical disease. Between December 2003 and September 2005, 38 patients with carcinoma of different locations in the head and neck region were irradiated. Five patients underwent concurrent chemotherapy (weekly cisplatin). Acute toxicity was evaluated according to Radiation Therapy Oncology Group toxicity scale for skin, mucous membrane, salivary glands, pharynx and esophagus and larynx. All 38 patients completed the therapy without urgency of interruption due to acute toxicity of radiotherapy. No patient experienced grade 4 toxicity. More severe toxicity was observed in patients with concurrent chemotherapy. The results confirm that the irradiation according to our SIB-IMRT protocol is a therapy with acceptable toxicity and there is a space for radiobiological enhancement of this regimen by concurrent chemotherapy, e.g. weekly cisplatin.