During a 4-yr period, ten of 87 patients (11.5%) who received therapeutic doses of radioactive iodine (I-131) for thyroid carcinoma developed acute and/or chronic sialadenitis involving the parotid (five patients) or submandibular (four patients) glands, or both (one patient). Nine of the 10 patients had received prior I-131 therapy; the precipitating I-131 dose varied between 10 and 164 mCi. Onset of symptoms occurred between 1 day and 6 mo following therapy and the duration varied from 3 wk to 21/2 yr. This complication occurs more often than has been appreciated.