Endometrial intraepithelial neoplasia (EIN) with secretory differentiation and ordinary EIN occurring in a secretory context are rare but recognized findings. We determined how often secretory differentiation in EIN was associated with evidence of circulating progestins in the background endometrium, and studied clinical characteristics and clinical outcomes of affected patients. We selected 41 patients with secretory differentiation in either the EIN itself (n=31) and/or background endometrium (n=38). Most (90%, 28/31) secretory EINs were associated with circulating progestins. Rare exceptions were observed, suggesting that secretory EIN may occur as a hormone-independent phenomenon. Circulating progestins are not sufficient, however, to induce EIN secretory differentiation, as 26% (10/38) of EIN within a secretory background were of the ordinary (non-secretory) type. EIN patients with secretory endometrium in the background are younger (averaging 45 years) than the aggregate group of all patients with EIN (53 years in previously published studies) and are often premenopausal with a cyclical source of endogenous progestins. Involution of EIN during follow-up was more frequent (81%, 17/21) for those with a secretory background at the time of initial EIN diagnosis compared with historical averages (25%, 36/142). These results suggest a potential role for endogenous progesterone, as well as therapeutic progestins, in modulating EIN outcomes.