Abnormalities of hematological and biochemical parameters are various and frequent during anorexia nervosa, and are mainly related to malnutrition, weight loss, and compensatory purgative behaviors. They are most often moderate and reversible through appropriate nutritional and weight rehabilitation, as well as well-conducted symptomatic treatment. Severe abnormalities are rarer, but are potentially serious or even fatal. Isolated moderate anemia and leukopenia are frequently noted, with thrombocytopenia being less frequent. Severe, bi-cytopenic, pancytopenic, and spinal cord injury are less common. They can be explained by the gelatinous transformation of the bone marrow caused by malnutrition. Biochemical abnormalities are typically hydroelectrolytic disorders (hypokalemia, hyponatremia, metabolic alkalosis), acute or chronic renal failure, elevated transaminases, risk of potentially severe hypoglycemia, and elevated lipid parameters. During the refeeding syndrome, hypophosphatemia is characteristic and may be associated with hypomagnesemia and hypocalcemia, and thiamine deficiency. Malnutrition can also lead to alterations in hormone status, including hypothyroidism, hypercorticism and hypogonadism, which may be involved in the development of serious bone conditions such as osteoporosis. These abnormalities should be routinely investigated, monitored, and corrected during anorexia nervosa. Early and multidisciplinary management of this eating disorder is essential to prevent chronicity of the disorder and the potential severity of these abnormalities.