Abstract Hereditary leiomyomatosis and renal cell carcinoma (HLRCC) is a familial cancer syndrome characterized by germline mutations in the fumarate hydratase (FH) gene that encodes the enzyme responsible for conversion of fumarate to malate in the Kreb's Cycle. HLRCC is associated with the development of cutaneous and uterine leiomyomas, and a highly aggressive form of type 2 papillary kidney cancer. HLRCC renal tumors can be extremely aggressive with patients demonstrating locally advanced or disseminated disease even when associated with small tumors 1-2 cm in size. Therefore, accurate identification of the “at risk” individuals is very important. In our experience, FH point mutations are identified in 90% of individuals that are clinically diagnosed with HLRCC, while in the remainder, FH may be inactivated by deletion or other causes. Several studies have reported complete deletions of the FH gene and a few have estimated the extent of these deletions and identified the additional genes that would be lost in conjunction with FH. Interestingly, families with complete FH deletion have demonstrated cutaneous and uterine leiomyomas but no families have yet been reported with evidence of kidney cancer. A single family has been identified that demonstrates a partial deletion resulting in the loss of exon 1, and in this case kidney cancer has identified within the family. In this study, patients with suspected HLRCC hereditary kidney cancer but no germline point mutation were enrolled on an NCI-IRB approved protocol and an Agilent custom high-definition comparative genomic hybridization (CGH) array was used to identify FH gene deletions. The assay identified complete germline deletion of a single copy of FH within seven suspected HLRCC patients deletion ranging from 249kb to 4.74Mb that in all cases resulted in a minimum addition loss of the KMO, OPN3, CHML, and WDR6 genes. All results were confirmed by CLIA approved tests and a further five new patients were identified by direct CLIA testing, including a novel partial deletion. In contrast to previous reports, seven out of twelve (58.3%) patients/families in our cohort demonstrated a personal or family history of HLRCC associated Type 2 papillary kidney cancer. In conclusion, complete or partial deletion of the FH gene is an infrequent but important cause of HLRCC and has no protective effect on the incidence of HLRCC associated type 2 papillary kidney cancer. Citation Format: Cathy D. Vocke, Christopher J. Ricketts, Lindsay A. Middelton, Youfeng Yang, W. Marston Linehan. Germline deletion of FH in hereditary leiomyomatosis and renal cell carcinoma (HLRCC). [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3179.