Objective: To assess the relationships between total body fat mass (FM) and hematological abnormalities in anorexia nervosa (AN). Method: Peripheral blood parameters and body composition were determined in 10 anorectic patients and 19 age-and sex-matched healthy subjects. In patients with AN, magnetic resonance imaging (MRI) studies of bone marrow were also performed. Results: Compared with controls, patients with AN had -41% body weight, -81% FM, -18.8% lean tissue mass (LTM), and -22.6% bone mineral content; they also had lower mean total leukocyte (4.52 ± 0.47 vs. 6.28 ± 0.33 × 10 3 /µl, p < .005), neutrophil (2.45 ± 0.34 vs. 3.46 ± 0.20 × 10 3 /µl, p < .005), monocyte (0.24 ± 0.03 vs. 0.37 ± 0.03 × 10 3 /µl, p < .05), and platelet counts (184 ± 17 vs. 238 ± 9 × 10 3 /µl, p < .005). Hemoglobin level was normal and comparable in both groups. In patients with AN, but not in controls, total leukocyte, neutrophil, eosinophil, and monocyte counts as well as hemoglobin level were highly correlated with FM expressed in absolute values or in percentage of body weight, but not with LTM. Moreover, AN patients with signal intensity patterns suggestive of serous atrophy of bone marrow at MRI had not only lower erythrocyte, leukocyte, neutrophil, and platelet counts, but they also had lower FM than AN patients with normal MRI patterns. Discussion: Hematological changes in AN, as assessed by peripheral blood parameters and MRI patterns of bone marrow, are correlated with total body FM depletion, suggesting that the reduction of adipose tissue adversely affects hematopoiesis.