Introduction: Water and electrolyte disorders are frequently found in oncologic patients since they are usually exposed to diverse symptoms, physiologic alterations, paraneoplastic syndromes, and drugs which predispose them to these disturbances. Material and method: A comprehensive review of this topic was performed carrying out a detailed analysis of the literature. Results: Any water and electrolyte alteration could be found in oncologic patients, and they add significant morbidity and mortality to this population. Conclusion: It is crucial to understand the underlying pathophysiologic mechanism of water and electrolyte disturbances in oncologic patients in order to achieve their correct diagnosis and adequate treatment.