The clinical history and biochemical and hematologic variables for 44 consecutive patients diagnosed with anorexia nervosa were recorded. Bone marrow aspirates and biopsy specimens were analyzed by standard morphologic procedures, and bone marrow adipocytes were studied morphometrically. The bone marrow of the 44 patients was classified as normal (5 cases [11%]), hypoplastic or aplastic (17 [39%]), with partial or focal gelatinous degeneration (13 [30%]), or with complete gelatinous degeneration of the bone marrow (GDBM; 9 [20%]). These patterns correlated with amount of weight loss (P = .005) but not other clinical findings. WBC counts were lower in patients with GDBM (P = .0189), but this and other peripheral blood variables did not always reflect the severity of bone marrow damage. Hypoplastic or aplastic bone marrow showed an increase in bone marrow fat fraction due to an increase in adipocyte diameters, while in GDBM, fat fraction and adipocyte diameters decreased. Morphologic changes in bone marrow and stereologic alterations in bone marrow adipocytes may be observed in anorexia nervosa. The extent of damage is related to the amount of weight loss, not to other factors. Peripheral blood cell counts may not reflect the extent of damage. In some patients, this process may be reversible with reestablishment of adequate nutritional intake.