A study was performed to identify variables that affected cause-specific survival (CSS) and local relapse-free rate (LRFR) in patients with differentiated thyroid cancer (DTC) and extrathyroid extension (ETE) and to examine the role of external beam radiotherapy (XRT). Prognostic factors were similar to those found in studies of all patients with DTC. In patients with postoperative gross residual disease treated with radiotherapy, 10-year CSS and LRFR were 48% and 90%. For patients with no residual or microscopic disease, 10-year CSS and LRFR were 92% and 93%. In patients older than 60 years with T3 ETE but no gross residual disease postoperatively there was an improved LRFR at 5 years of 96%, compared to 87.5% without XRT (P = .02). Patients with gross ETE benefit from XRT and there may be a potential benefit in reducing locoregional failure in patients over 60 years with minimal extrathyroidal extension (T3).