Therapeutic irradiation to the head and neck for cancer damages salivary glands present in the radiation field. Despite long recognition of radiation-induced salivary hypofunction, and the associated oral morbidities, the specific damage mechanism(s) is not known and the structure and functional integrity of the surviving parenchymal tissue has not been well-documented. Detailed knowledge of the latter is particularly necessary in order to design appropriate corrective therapies. It is the purpose of this study to provide such a detailed structural and functional assessment of human parotid glands following irradiation. The study will examine 20 patients beginning just prior to therapeutic irradiation and continuing at intervals for 3 years for a total of 5 study visits. Study visits (prior to irradiation and at 4 weeks, 12 weeks, 12 months and 36 months post-irradiation) will include the following procedures: i) detailed oral exam and structured interview; ii) salivary gland functional assessment; iii) sialography of each parotid gland; iv) 99mTcO4 scan of the salivary glands; and v) a magnetic resonance imaging (MRI) scan of the parotid glands. Based on previous single observation studies in humans, and more detailed animal studies, we hypothesize that ionizing radiation will lead to reduced parotid gland function and diminished salivary parenchymal tissue (with a preferential loss in acinar versus ductal cells). Further, we hypothesize that the parenchymal loss will increase with time (replaced by fat and connective tissue) and lead to progressive irreversible salivary dysfunction.