Abstract Cystic lymphangioma are rare benign vascular or lymphatic tumors, diagnosed mostly in newborns or children, that may become life-threatening because of local invasiveness. Surgical “en-bloc” resection with negative margins is the only curative treatment, but some patients are diagnosed with unresectable tumors. We describe the case of a young adult with giant unresectable mesenteric lymphangioma. Extensive pathological characterization as well as whole exome and transcriptome sequencing enabled us to identify mTOR pathway activation within endothelial tumor cells. The patient was treated with everolimus and experienced major partial response, leading to the surgical resection of the residual lesions. This case highlights the importance of molecular characterization of adult cystic lymphangioma for mTOR pathway activation because multidisciplinary therapeutic approaches, including neoadjuvant everolimus and secondary surgery, can lead to complete cure of this rare condition. Key Points