The effect of an adjuvant mistletoe extract treatment was tested in a prospective, randomised controlled clinical trial involving 477 patients with head and neck squamous cell carcinoma. The patients were stratified into two treatment groups that underwent surgery or surgery followed by radiotherapy and both groups were randomised for additional treatment with mistletoe extract. Patients treated with a mistletoe lectin-1 (ML-1) standardised mistletoe preparation had no lower risk of local/locoregional recurrences, distant metastases or second primaries. In the main analysis based on 202 patients treated with surgery and 275 patients treated with surgery and radiotherapy the adjusted hazard ratio for the disease-free survival (DFS) was 0.959 (95% confidence interval (CI) 0.725-1.268). The 5-year survival rates of patients from the mistletoe group were no better than the survival rates of patients from the control group. Furthermore, no significant changes in the cellular immune reaction or in quality of life could be detected. We conclude that the used mistletoe preparation has no indication in the adjuvant treatment of patients with head and neck cancer.