The aim of this study was to investigate cell cycle changes during radiation treatment and establish whether treatment intervention could be considered if these changes helped to predict outcome. 33 patients with head and neck cancer were administered iododeoxyuridine (IdUrd) prior to treatment and a second administration of bromodeoxyuridine (BrdUrd) prior to the fifth fraction of 2 Gy. Biopsies were taken several hours after each injection and flow cytometry was used to calculate changes in the cellular kinetics and cell cycle delay in vivo. The kinetic response of the tumour cells was variable; some showed an increase in proliferation during the first week of treatment, whilst the majority showed an inhibition of proliferation. Reduction in the labelling index (LI) and the pretreatment DNA ploidy status and not delays in G2 were the only parameters to correlate with clinical outcome. A lack of reduction in the LI after 1 week of radiotherapy and DNA aneuploidy predicted a group of patients where radiotherapy failed. This information could be helpful in planning future treatment interventions.