Bulimia Nervosa (BN) is a serious eating disorder, which affects 0.8-2.9% of the young population. The etiology is unknown and biomarkers would support in understanding the pathophysiology of BN, and in identifying BN patients that may benefit from medical treatment. This systematic review aims to answer whether (a) BN deviate from healthy controls in terms of serotonin (5-HT) biomarkers in blood, and whether (b) blood-based 5-HT biomarkers could be used to tailor psychopharmacological treatment in BN. A literature search using PubMed, PsycINFO and Embase was done using the following search terms: "Bulimia Nervosa" AND "serotonin" AND "blood" OR "plasma" OR "serum". 32 studies were included in this systematic review. Several biomarkers and challenge tests were identified and all studies described an association with BN and dysregulation of the 5-HT system compared to healthy controls. Several studies pointed to an association also to borderline symptoms in BN. BN deviate from healthy controls in terms of 5-HT biomarkers in blood supporting an abnormal 5-HT system in BN. 5-HT biomarkers and associated methods could be used to tailor treatment in BN although as yet, most tests described are unpractical for bedside use.