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Diagnosis of endometrial intraepithelial neoplasia (EIN) requires learning new criteria. Two trainees rendered diagnoses based on biopsy findings, and then measured the effect of reviewing PAX2 on their interpretation. Fifty-two endometrial biopsy specimens diagnosed as having EIN were evaluated using EIN criteria. Background endometrial pattern, altered differentiation, and any features complicating diagnosis were noted. PAX2 stains were scored as confusing, helpful, or noncontributory. Fifty-two cases generated 104 passes; 82% were rediagnosed as EIN. The diagnosis was complicated because of altered differentiation (14%), EIN and background separation (13%), large lesions lacking background (11%), and secretory background (8%). PAX2 was most helpful in cases with secretory backgrounds and when EIN lacked adjacent normal tissue, and most confusing when scoring was ambiguous (14%). The diagnosis of EIN can be difficult when: (1) the lesion cannot be easily compared with background; (2) there is a confounding process; and (3) gland differentiation is altered. PAX2 can be of assistance in delimiting EIN lesions.