The effects of increased amounts of progesterone on the endometrium, including such features as eosinophilic cytoplasmic metaplasia, glandular atrophy, and decidualized stroma, are well-known among surgical pathologists. These changes are typically seen as secondary effects of pregnancy or exogenous hormone therapy for birth control purposes or abnormal bleeding. Treatment with progesterone has become a viable alternative to hysterectomy in some patients with complex atypical hyperplasia (CAH) and well-differentiated endometrial carcinoma (WDC), especially those who are poor surgical candidates or those wishing to preserve fertility. To date, only 1 study has specifically examined the effects of progestin therapy on patients with a previous diagnosis of CAH or WDC. That study proposed a classification scheme for the assessment of treated CAH and WDC. The authors concluded that after 6 months of treatment, endometrial biopsy findings of persistent cytologic atypia and architectural abnormalities were associated with treatment failure. This current study aims to assess the previously proposed criteria in a cohort of 30 patients (18 with a diagnosis of CAH and 12 with a diagnosis of WDC), and determine the usefulness of these criteria in clinical practice. Our study confirms that cytologic atypia after 6 months of therapy is strongly associated with treatment failure, and should be an indication to pursue definitive surgical treatment in these patients.