Introduction: Hyperthermia is a complementary therapy in oncology having various pros and contras for its application. Ascites, pleural effusion, edema and other electrolyte accumulations are frequently excluded from the treatability of the patients with heating locally or systemically. The special gathering of electrolytes is sometimes contraindicated, at times not mentioned in the clinical protocols. However, it is certainly challenging in the oncology where micro and macro edemas, as well as larger electrolyte accumulations (e.g. ascites, pleural effusion), are very frequent. Methods: Excluding patients with accumulation of free electrolytes limits the applications of hyperthermia. To find a solution we are studying the microvasculature and fluid dynamism together with the electric field effects, including the injury currents. The hyperthermia method which we investigate is the modulated electro-hyperthermia (mEHT). We use the Starling's equation and the injury current in the frame of non-equilibrium thermodynamics and in connection with the biologically closed electric circuits. Results: It is shown that mEHT, unlike the conventional hyperthermia, is applicable for patients who have edema and other free-electrolytes in the volume which is targeted. The heterogeneous heating (unlike the homogeneous, isothermal conventional hyperthermia) promotes the development of tumor-specific immune actions, and so has less adverse-effects, and longer survival time for patients in advanced, metastatic cancers too. Conclusion: mEHT is well applicable in cases of ascites, pleural effusion, edema and other electrolyte accumulations when a patient is treated in complex (complementary) oncological therapy.