Abstract Background and purpose The aim of this report was to describe the incidence and prevalence of acute and late morbidity in the DAHANCA 6&7 multicentre randomised trial with accelerated radiotherapy for squamous cell carcinoma of the head and neck. Materials and methods The DAHANCA 6&7 study included 1476 patients eligible for primary radiotherapy alone. Patients were randomised between five or six weekly fractions of conventional radiotherapy. The prescribed dose was 66–68 Gy in 33–34 fractions. All patients were seen weekly during treatment and at regular intervals after completion where detailed morbidity recording was done. Reports from 1468 patients were available for analysis of treatment related morbidity. Results Accelerated radiotherapy caused a significant ( p < 0.05) increase in the peak incidence of: use of analgesics (53% vs. 65%), dysphagia (35% vs. 45%), mucosal oedema (52% vs. 59%), and mucositis (33% vs. 53%). All acute reactions were reversible and healed within three months after radiotherapy. Loss of taste, xerostomia, and acute skin reaction was not different between the two groups. For all late endpoints except fibrosis and atrophy a decline in prevalence was observed in the years after radiotherapy, there was no significant difference between randomisation arms in any of the late endpoints. Conclusions Six fractions per week, resulting in a one-week reduction in overall treatment time relative to conventional radiotherapy increased acute but not late morbidity. Since acceleration improves loco-regional tumour control, the schedule represents a significant improvement of the therapeutic ratio for head and neck radiotherapy and might be close to the maximal gain possible with accelerated fractionation alone.