Taste loss is a major cause of morbidity in patients undergoing head and neck irradiation. Previous studies have reported the alteration of the four basic tastes in patients with head and neck cancer during radiotherapy. However, only a few studies have been conducted on the effects of irradiation on the function of umami taste, a novel and basic taste recently recognized. In a prospective study, 52 patients undergoing radical head and neck irradiation were assessed for taste loss. Taste ability was measured by the taste threshold for umami quality using the whole-mouth taste method in patients before, during, and immediately after radiotherapy. Umami taste declined of the 3rd week after the start of radiotherapy and improved of the 8th week.