['Familial nonmedullary thyroid carcinoma (FNMTC) is frequently detected, but the prevalence or the aggressiveness of FNMTC is still unclear. We aimed to investigate the prevalence, clinical characteristics, and prognosis of FNMTC.', 'This study included 3056 nonmedullary thyroid carcinoma (NMTC) patients who were pathologically confirmed to exhibit differentiated thyroid carcinoma from January 1962 through March 2010. The duration of follow-up was 6.2±6.2 years.', 'The prevalence of FNMTC was 9.6%; 37.9% of the FNMTC patients exhibited a parent-offspring relationship, and 62.1% exhibited a sibling relationship. FNMTC was smaller in tumor size (1.2±0.9 vs. 1.4±1.1\u2009cm) and more multifocal (33.6% vs. 27.0%) than sporadic cases. FNMTC presented higher recurrence rates (29.5% vs. 19.8%) and shorter recurrence-free survival than sporadic NMTC (p=0.046). When we compared sporadic NMTC with parent-offspring or sibling FNMTC separately, parent-offspring FNMTC was more multifocal (39.3% vs. 27.0%), while sibling FNMTC was more prevalent in female patients (89.6% vs. 82.5%) and presented smaller tumors (1.2±0.8 vs. 1.4±1.1\u2009cm) than sporadic NMTC. The recurrence rate was higher than that of sporadic NMTC in parent-offspring FNMTC (35.6% vs. 19.8%) but not in sibling FNMTC. Among the 123 parent-offspring FNMTC cases, the second generation exhibited an earlier age at the diagnosis (38±11 vs. 57±11 years), more extrathyroidal invasion (57.8% vs. 29.4%), a higher recurrence rate (50.0% vs. 19.0%), and shorter recurrence-free survival (p=0.015) than the first generation.'] Are the long-term outcomes of the second generation of familial nonmedullary thyroid carcinoma more aggressive than sporadic cases?