Few simple clinical measures are available to monitor swallowing performance in head and neck cancer. Water swallow tests (WST) have been used as a part of clinical assessments in neurological dysphagia. The aim of this paper is to evaluate the utility of the 100 mL WST in head and neck cancer patients. The 100 mL WST was performed on 167 head and neck cancer patients. Measures were compared with respect to tumor site/stage, gender and age. The cohort was compared with published healthy controls. The test was quick to administer with excellent compliance. Patients had significantly poorer swallows than the published control group (mean reduction 1.6 mL/s). Function worsened with increased tumor stage and for patients with pharyngeal tumors. The 100 mL WST is an effective swallowing performance measure and is easily incorporated into a clinical examination. This paper provides benchmark data on the 100 mL WST for individuals with head and neck cancer.