The aim of this work is to analyse the proliferation of tumour cells in the treatment gapduring the radiotherapy for head neck cancer.Material and Methods: The clinical material is based on records of head and neck patients treated by radiotherapyalone in our institution. The effect of radiotherapy was assumed to be described by a linearquadratic model. The patient data were fitted directly to the radiobiological model and theparameters were estimated using maximum-likelihood procedures.Results: According to our model results of treatment were significantly correlated with Normalised Total Dose of radiation, the tumour progression (according to TNM), the overall treatment time and the gap duration. The laryngeal cancers had better prognosis then cancers of oroand nasopharynx. When the treatment time is prolonged without treatment interruptions 0.36 Gylday is lost due to the repopulation of tumour cells. During the treatment gap proliferation is faster and 0.67 Gylday is lost.Conclusion: Proliferation of tumour cells is faster during the treatment gap then during the days with irradiation.