A multicenter phase II trial was conducted to define the activity of letrozole in postmenopausal women with recurrent or advanced endometrial carcinoma, who had no more than one prior line of progestins and never had chemotherapy (except adjuvant). Archival paraffin-embedded tumor samples were retrieved to determine the expression level of estrogen (ER) and progesterone receptor (PgR), p53, HER-2, bcl-2 and PTEN protein, and phosphorylation status of protein kinase B (PKB/Akt). Thirty-two eligible patients were treated with letrozole at 2.5 mg daily continuously, of whom 10 (31%) had prior progestins. Of the 28 patients evaluated for response, one complete and two partial responses were noted; overall response was 9.4% (95% confidence interval 2-25%). Eleven patients had stable disease for a median duration of 6.7 months (range 3.7-19.3 months). Amongst 22 patients who had tumor blocks available, the proportion showing positive expression of the following markers includes: PgR (86%), ER (86%), PTEN (82%), phosphorylated PKB/Akt (59%), bcl-2 (45%), p53 (32%), and HER-2 (0%). None of these markers correlated with response to letrozole or disease progression. In conclusion, letrozole is well tolerated but has little overall activity in this cohort of women with endometrial cancer.