PURPOSE: To compare outcomes of treated vs untreated Liver Imaging Reporting and Data System category 4 (LR-4) masses after transcatheter arterial embolization. MATERIALS AND METHODS: In 167 patients undergoing embolization for HCC from January 2005 to December 2012, LR-4 masses were retrospectively identified on CT and MR imaging examinations performed before embolization. In 149 patients undergoing embolization from January 2013 to December 2016, masses prospectively classified as LR-4 were identified. In total, there were 81 LR-4 masses in 62 patients (16 women; mean age 62 y; range 29-83 y). Procedures were reviewed to determine whether LR-4 masses were within or outside the liver volume that received embolization during treatment of dominant masses. Time to progression to LR-5 and by modified Response Evaluation Criteria in Solid Tumors (mRECIST) was estimated for treated vs untreated LR-4 masses using the Kaplan-Meier method and compared using the log rank test. RESULTS: LR-4 masses averaged 1.8 cm; 88%, 60%, 14%, and 14% demonstrated arterial phase hyperenhancement, washout, a capsule, and growth. Of LR-4 masses, 62 were within the liver volume that received embolization and considered treated, and 19 were outside and considered untreated. Response rates according to mRECIST were 37% vs 21% for treated vs untreated masses (P = .27). The 6- and 12-month rates of progression to LR-5 were 7% and 26% for treated masses vs 27% and 75% for untreated masses (P = .001). According to mRECIST, 7% and 27% of treated masses progressed vs 30% and 65% of untreated masses (P = .001). CONCLUSIONS: LR-4 masses that receive embolization in the setting of dominant masses elsewhere show lower rates of progression compared with untreated masses.