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We performed prospective immunohistochemical analysis of LAG3 for 430 consecutive patients with advanced gastrointestinal, genitourinary, or rare cancers between June 2012 and March 2016. Most patients (428/430, 99.5%) were evaluable for LAG3 expression by immunohistochemistry. In total, 18.5% (79/428) of the evaluated cancers expressed LAG3, including pancreatic (33.3%, 2 of 6), gastric (24.7%, 21 of 85), colorectal (23.6%, 48 of 203), melanoma (12.5%, 1 of 8), genitourinary (9.5%, 4 of 46), biliary tract (6.3%, 1 of 16), and sarcoma (5.4%, 2 of 37), but not miscellaneous (0.0%, 0 of 14) or hepatocellular (0.0%, 0 of 15) cancer. Among 149 metastatic CRC patients, there was no statistically significant difference in gender, age, primary tumor site, pathologic differentiation, KRAS and NRAS status, BRAF status, and microsatellite instability according to LAG3 status (expressed vs. non-expressed). Among 53 metastatic GC patients, LAG3 was only significantly associated with EBV status (P = .042). Our results add to the emerging literature on LAG3 expression in various cancer types and support the need for extended clinical exploration of this target for immunotherapy. Citation Format: Su Jin Lee, Sun-ju Byeon, Jeeyun Lee, Se Hoon Park, Joon Oh Park, Young Suk Park, Won Ki Kang, Ho Yeong Lim, Kyoung-Mee Kim, Seung Tae Kim. LAG3 in Solid Tumors as a Potential Novel Immunotherapy Target [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1107.