CONTEXT: There are no known effective and reliable biomarkers to distinguish benign thyroid nodules from papillary thyroid carcinomas (PTC). Previous studies have indicated that serum microRNA (miRNA) profiles may be diagnostic and/or prognostic markers for numerous other cancers. OBJECTIVE: We studied circulating miRNA profiles in patients with PTC or benign nodules and healthy controls to identify serum miRNA that may be useful as markers for PTC. DESIGN, SETTING, AND PARTICIPANTS: Genome-wide serum miRNA expression profiles were determined using Solexa sequencing followed by extensive quantitative RT-PCR validation in 245 subjects (106 patients with PTC, 95 patients with benign nodules, and 44 healthy controls). A panel of miRNA was used to assess the expression of specific miRNA in the sera and thyroid tissues of patients with PTC or benign nodules. RESULTS: The expression of serum let-7e, miR-151-5p, and miR-222 was significantly increased in PTC cases relative to benign cases and healthy controls. Receiver operating characteristic curve analyses indicated that use of these three miRNA had a high diagnostic sensitivity and specificity for PTC. Serum let-7e, miR-151-5p, and miR-222 levels were found to be well correlated with certain clinicopathological variables, such as nodal status, tumor size, multifocal lesion status, and Tumor-Node-Metastasis stage. Expression of serum miR-151-5p and miR-222 in a subset of PTC patients decreased significantly after tumor excision. Increased expression of miR-151-5p and miR-222 was also found in the tissue of PTC patients. CONCLUSIONS: Our study demonstrates that serum miRNA profiles may be used as novel and minimally invasive diagnostic markers for PTC.