purpose: to report the finding of nasolacrimal drainage system obstruction associated with radio iodine therapy and to review clinical data and the surgical treatment outcome of this rare complication. methods: we retrospectively analyzed ophthalmological data of patients with history of thyroid carcinoma that underwent radioactive iodine i-131 therapy and were referred to lacrimal surgery. results: 17 patients with thyroid cancer treated with thyroidectomy and radioactive iodine i-131 therapy presented symptomatic nasolacrimal duct obstruction after 13.2 months following cancer treatment. 11 patients presented bilateral epiphora, 8 had lacrimal sac mucocele. age range was 30 to 80 years, 10 patients had less than or equal to 49 years. the mean cumulative dose of radioiodine was 571mci (range: 200-1200 mci). nasal obstruction symptoms and increased salivary glands were also present in 53% of patients. all subjects underwent dacryocystorhinostomy. dilation of the lacrimal sac and increased intraoperative bleeding was also observed in 3 younger patients. complete epiphora and dacryocystitis resolution after surgery occurred in 82.4%, and partial in 17.6% (3 patients that still presented unilateral relapse after correction of bilateral obstruction). mean follow-up was 6 months (range: 2-24 months). conclusions: cumulative high dose of radioidine, nasal and salivary gland dysfunction are associated with lacrimal drainage obstruction. we observed a great percentage of younger patients presenting dacryocystitis when compared to the idiopathic dacryostenosis. radioactive iodine uptake by nasolacrimal duct mucosa with subsequent inflammation, edema and fibrosis seems to have a relationship to lacrimal duct obstruction. the knowledge of this complication is important for the study and proper management of these patients.