Follow-up biopsies in patients with progestin-treated atypical endometrial hyperplasia/endometrioid intraepithelial neoplasia (AH/EIN) may show papillary structures, the significance of which is unclear. Methods: The authors reviewed 253 serial specimens of 84 consecutive patients diagnosed with AH/EIN, inclusive of each patient's pre-progestin treatment sample and all post-treatment specimens. We assessed the predictive relationship between papillary architecture in a post-treatment biopsy and two study outcomes: AH/EIN or carcinoma in at least one sample subsequent to the one in which papillae were identified, and/or the last specimen received for that patient. Results: Papillae were identified in only 51.5% of pre-treatment samples but were present in at least one subsequent post-treatment sample for all patients. Post-treatment samples that exhibited papillae and no glandular crowding were associated with AH/EIN in at least one subsequent specimen in 39.7% (29/73) of cases, compared to 24.0% (6/25) in samples with neither papillae nor glandular crowding (p = .227) and 64.0% (16/25) in samples with concurrent gland crowding and papillae (p = .048). Univariate logistic regression analyses showed that the presence of papillae was not associated with study outcomes (odds ratio [OR], 0.99; 95% confidence interval [CI], 0.49 to 1.99; p = .985), as compared with gland crowding (OR, 1.54; 95% CI, 1.04 to 2.27; p = .031), or concurrent papillae and gland crowding (OR, 2.36; 95% CI, 1.01 to 5.52; p = .048). Conclusions: In post-treatment samples of progestin-treated AH/EIN, the presence of papillary architecture was not demonstrably associated with study outcomes independent of gland crowding, although the concurrent presence of both features may be significantly predictive.