The extent to which this prognosis depends on their sensitivity to adjuvant treatment is unknown, as is the optimal therapy for advanced-staged or recurrent <i>POLE</i>-mutant cancers.<b>Experimental Design:</b> We examined the recurrence-free survival of women with <i>POLE</i>-mutant and <i>POLE-</i>wild-type endometrial cancers (EC) in the observation arm of the randomized PORTEC-1 endometrial cancer trial (<i>N</i> = 245 patients with stage I endometrial cancer for analysis).