Purpose: In gastrointestinal stromal tumors (GIST), lipid-metabolizing enzymes remain underexplored, including fatty acid synthase (FASN).Experimental Design: Forty GISTs were quantitated for FASN mRNA abundance. FASN immunoexpression was informative in 350 GISTs, including 213 with known KIT/PDGFRA/BRAF genotypes. In imatinib-resistant FASN-overexpressing GIST cells, the roles of overexpressed FASN and FASN-targeting C75 in tumor phenotypes, apoptosis and autophagy, KIT transcription, PI3K/AKT/mTOR activation, and imatinib resistance were analyzed by RNAi or myristoylated-AKT transfection. The therapeutic relevance of dual blockade of FASN and KIT was evaluated in vivoResults:FASN mRNA abundance significantly increased from very low/low-risk to high-risk levels of NCCN guidelines (P < 0.0001). FASN overexpression was associated with a nongastric location (P = 0.05), unfavorable genotype (P = 0.005), and increased risk level (P < 0.001) and independently predicted shorter disease-free survival (P < 0.001). In vitro, FASN knockdown inhibited cell growth and migration, inactivated the PI3K/AKT/mTOR pathway, and resensitized resistant GIST cells to imatinib. C75 transcriptionally repressed the KIT promoter, downregulated KIT expression and phosphorylation, induced LC3-II and myristoylated AKT-suppressible activity of caspases 3 and 7, attenuated the PI3K/AKT/mTOR/RPS6/4E-BP1 pathway activation, and exhibited dose-dependent therapeutic additivism with imatinib. Compared with both monotherapies, the C75/imatinib combination more effectively suppressed the growth of xenografts, exhibiting decreased KIT phosphorylation, Ki-67, and phosphorylated PI3K/AKT/mTOR levels and increased TUNEL labeling.Conclusions: We have characterized the prognostic, biological, and therapeutic implications of overexpressed FASN in GISTs. C75 represses KIT transactivation, abrogates PI3K/AKT/mTOR activation, and provides a rationale for dual blockade of KIT and FASN in treating imatinib-resistant GISTs. Clin Cancer Res; 23(16); 4908-18. ©2017 AACR.