Osteoporosis is one of the major complications in anorexia nervosa (AN) patients. Receptor activator of nuclear factor κB ligand (RANKL) and osteoprotegerin (OPG) have been identified as important regulators of bone turnover. The objective of this study was to clarify the role of RANK-RANKL-OPG system, and their relationship with other regulators for bone metabolism in AN patients. We investigated serum levels of RANKL, OPG, and bone turnover markers of 26 Japanese young female AN patients and 7 age-matched healthy women. We measured serum levels of estradiol (E2), insulin like growth factor-I (IGF-I) and triiodothyronin (T3) from the same samples and studied their relationship with RANKL or OPG. Mean serum levels of E2, IGF-I, T3 and leptin in AN patients were significantly lower than those of controls (p<0.05). Serum levels of OPG in AN patients were significantly higher than those in controls and negatively correlated with body mass index (BMI), E2, IGF-I or leptin. Serum levels of free RANKL could not be detected except for only one healthy control in both groups. These results suggest that serum OPG levels may be increased by a compensatory mechanism for malnutrition and estrogen deficiency which induces an increase in bone resorption.