Radiotherapy (RT) used for head and neck cancers causes permanent salivary gland dysfunction (SGD). Previous short-term studies have demonstrated that pre-RT salivary flow rates and the amount of radiation exposure to parotid glands influence the amount of RT-induced SGD. The purpose of this study was to determine which variables are related to the development of long-term post-RT SGD. Parotid flow rates (PFR) were assessed prior to and 1 year after completion of RT in spared parotid glands from 34 patients from 2 parotid-sparing protocols. The results reveal that spared PFR were not significantly higher 1 year post-RT in patients who had high pre-RT PFR, when compared with patients with low pre-RT PFR. However, patients who received higher doses of RT to spared parotid glands had lower PFR 1 year post-RT, compared with patients who had received lower doses of RT. These one-year findings suggest that high pre-RT PFR do not provide protection against RT-induced SGD. Conversely, reduced RT dosages to contralateral parotid glands are protective of PFR after completion of RT.