5503 Background: While a number of potentially active chemotherapeutic agents are available for the treatment of endometrial cancer, some therapies may not be effective in all women. An in vitro assay performed before therapy initiation to identify the drug(s) most likely to be effective for the individual patient would have clinical utility. The primary objective of this study was to determine whether the results from an in vitro chemoresponse assay are similar to the published population response rates for endometrial cancer. METHODS: Tumor specimens were collected from Dec 1, 2007 to July 15, 2008 from 405 consecutive patients with endometrial carcinoma and were tested in vitro for a response by using the ChemoFx assay. Tumors were categorized prospectively as responsive (R), intermediately responsive (IR) or nonresponsive (NR) to each drug or combination tested. The in vitro response rates were compared to reported response rates from clinical trials. RESULTS: FIGO stage distribution was 171 stage I patients, 32 stage II patients, 106 stage III patients, 57 stage IV patients, 37 recurrent patients, and 2 unknown patients. The assay was successfully completed for 360 (89%) cases. The majority of tumors (73%) exhibited varying degrees of responsiveness to different drug(s). No significant difference in response rate was observed between primary and recurrent tumors or between stage I/II and III/IV tumors. CONCLUSIONS: In vitro tumor response rates were similar to reported treatment response rates for all treatments except single-agent carboplatin. A drug response marker can provide clinically useful information to optimize individual chemotherapy regimens for women with endometrial cancer. [Table: see text] [Table: see text].