Pediatric differentiated thyroid cancer (DTC) is a rare disease. Initial treatment of DTC consists of a total or near-total thyroidectomy and 131 I therapy. Previous studies on adults showed that 131 I treatment may reduce salivary gland function (SGF). Studies regarding SGF in children treated for DTC are sparse. Our aim was to assess the long-term effects of 131 I treatment on SGF in survivors of pediatric DTC. Methods: In a nationwide cross-sectional study, SGF in patients treated for pediatric DTC between 1970 and 2013 (.5 y after diagnosis, $18 y old at the time of evaluation) was studied. SGF was assessed by sialometry, sialochemistry, and a xerostomia inventory. Salivary gland dysfunction (SGD) was defined as an unstimulated whole saliva flow of no more than 0.2 mL/min or a stimulated whole saliva flow of no more than 0.7 mL/min. Results: Sixty-five patients underwent 131 I treatment (median age at evaluation, 33 y, with an interquartile range [IQR] of 25–40 y; 86.2% female; median follow-up period, 11 y, with an IQR of 6–22 y). Median cumulative 131 I activity was 5.88 GBq, with an IQR of 2.92–12.95 GBq, and 47.7% underwent multiple 131 I administrations. SGD was present in 30 (47.6%) patients. Levels of amylase and total protein in saliva were reduced. Moderate to severe xerostomia was present in 22 (35.5%) patients. Stimulated salivary secretion was lower and the severity of xerostomia complaints higher in patients treated with higher cumulative 131 I activity. Conclusion: In survivors of pediatric DTC, clinically significant SGD was found in 35.5% and was related to the cumulative 131 I activity of the treatment.