Unstimulated and stimulated salivary flow rates, Streptococcus mutans samples, and dental caries data were obtained from 50 patients with thyroid cancer who had received radioactive iodine, I131, from 0.3 to 20 years earlier. The salivary flow rates were compared with a healthy control group, and the S. mutans counts were compared to a group of patients with head and neck cancer who were sampled before radiotherapy. Flow rates were found to be significantly lower in the patients with thyroid cancer, and S. mutans levels were slightly but not significantly higher than the controls. Longitudinal flow rate data taken on four patients, who served as their own controls before and after I131 therapy, indicated a trend in saliva reduction.