AbstractObjectiveThe utility of bioelectrical impedance analysis (BIA) in patients with anorexia nervosa (AN) is imperfectly defined. Furthermore, any advantage accrued by BIA with vector analysis (BIVA) is unknown.MethodWe conducted a retrospective analysis of 57 women with AN admitted to our service who underwent BIA and BIVA. Twenty‐seven women were observed during short‐term (3 weeks) and 13 women during longer‐term (3 months) weight gain.ResultsBioelectrical impedance analysis produced implausible results in 47% of the patients. BIVA demonstrated low body cell mass and highly variable extracellular water (ECW) volume, ranging from volume contraction to volume expansion on admission and during treatment. BIVA suggested that short‐term weight gain predominantly consisted of ECW volume, whereas longer‐term weight gain resulted in increased hydrated body cell mass.ConclusionConventional BIA has little utility in these patients. However, BIVA could be a suitable alternative in the medical management reflecting ECW volume changes and later genuine tissue mass increases. Copyright © 2012 John Wiley & Sons, Ltd and Eating Disorders Association.