Purpose A few recent studies have found elevated ferritin levels in patients with anorexia nervosa (AN), indicating ferritin as a potential biomarker of disease severity. The purpose of this study was to study how body mass index (BMI) and changes in BMI affect plasma ferritin concentrations in Swedish patients with eating disorders. Materials and methods In a retrospective computer search from 2009 to 2014, 662 patients with an eating disorder were identified from more than 200,000 individuals with electronic medical records. Three hundred and eighty-nine patients (374 females and 15 males) were found to have at least one p-ferritin value with a corresponding BMI value. Patients with AN were compared to a combined group consisting of patients with bulimia nervosa (BN) and patients with an eating disorder not otherwise specified (EDNOS). Results Patients with AN had lower BMI compared to the combined group of patients with other eating disorders (BMI = 16.5 +/- 1.5, n = 77 vs. 21.0 +/- 4.7, n = 312, p amp;lt; 0.001). Patients with AN also had higher plasma ferritin levels (median 42 mu g/L (range 3.3-310) vs. 31 mu g/L (range 2.8-280); p amp;lt; 0.001). As BMI increased in patients with AN, ferritin levels decreased (from a median of 40 mu g/L (7-400) to 26 (4-170), n = 47; p amp;lt; 0.001). Discussion Measuring ferritin in patients with AN could be valuable in monitoring improvements of nutritional status, but the full clinical value of following ferritin in individual patients has yet to be determined. The study also shows how research can benefit from electronically captured clinical data using electronic health records.