Introduction. Central pontine myelinolysis is characterized by the occurrence of acute demyelinating lesions of cells in the pons secondary to abrupt oscillations of serum osmolarity. Its exact incidence is not well defined, but studies show a prevalence of 0.25 to 0.5% in the general population, 2.5% in the intensive care unit, and up to 10% in patients with risk factors, such as chronic liver disease and hepatic transplantation, alcoholism, malnutrition, diuretic therapy, electrolyte imbalance, hypoglycemia, and hyperglycemia. Case Report. A 70-year-old white female with extranodal diffuse large B-cell non-Hodgkin's lymphoma (extensive mass on the left anterior chest wall), stage IVA, developed pontine myelinolysis secondary to hypovolemic acute hypernatremia, which occurred due to diarrhea caused by chemotherapy (rituximab, cyclophosphamide, doxorubicin, and vincristine). Discussion. Pontine myelinolysis occurs most often due to the rapid correction of chronic hyponatremia. But here, we describe a case of the disease secondary to the occurrence of hypovolemic acute hypernatremia in a patient with a hematological malignancy under treatment, who was on chronic treatment with thiazide diuretics and who presented with other electrolyte disturbances as risk factors for the development of pontine myelinolysis.