Metabolic disorders have been observed in patients with hematological malignancies, especially after medical treatment (chemotherapy, corticosteroids, radiation, hormonal agents, and biological response modifiers) as well as following bone marrow and stem cell transplantation. Hyperuricemia is the most common metabolic abnormality; hyperuricaemia less commonly may be associated with hyperkalemia, hyperphosphatemia and hypocalcemia, in the framework of an oncologic emergency that is the Tumor lysis syndrome. Hypercalcemia is relatively common in patients with multiple myeloma and adult T-cell Lymphoma. Cases of Syndrome of inappropriate secretion of antidiuretic hormone (SIADH) in patients with hematological malignancies have also been reported. Idiopathic hyperammonemia may occur in oncohematological patients after receiving intensive chemotherapy or following bone marrow transplantation. Moreover, there is evidence that patients with lymphoma, leukemia and multiple myeloma can develop type B lactic acidosis. Non–islet cell tumor hypoglycemia and hyperglycemia are other potential metabolic abnormalities occurring in patients with hematological malignancies. The pathogenesis of these metabolic disorders is often unclear and several theories have been postulated; possible mechanisms include: increase in neoplastic cell turnover and apoptosis, blast crisis, cytotoxic effects of chemotherapy, tumor secretion of hormones, peptides or cytokines, immune cross-reactivity between malignant and normal tissues, malignancy-induced enzyme dysfunction. Parenteral nutrition, sarcopenia, cachexia, stress, immune deficiency and infections could contribute. Some pathological conditions, such as diabetes mellitus, renal failure, chronic obstructive pulmonary disease, cardiovascular dysfunction, liver disease and other disorders, may predispose oncohematological patients to develop metabolic abnormalities; on the other hand the occurrence of hematological malignancies can worsen pre-existing dysmetabolic conditions, that increase the outcomes of these patients. Although successful treatment of the underlying tumor often improves metabolic disorders, these conditions often worse prognosis and are associated with poor survival; thus it is important to consider early detection and effective treatment.