The invention relates to medicine and veterinary science and can be used for treating mainly solid tumors. The inventive method for treating oncological diseases consisting in injecting a blood extracellular DNA destroying agent into a systemic blood circulation. Said blood extracellular DNA destroying agent can be embodied in the form of a DNAse enzyme, in particular a bovine pancreatic DNAse which is parenterally injected in doses ranging from of 50 000 Kunz units to 250 000 000 Kunz units a day every day during 5-360 days or in the form of a recombinant human DNAse. In addition, a blood extracellular DNA binding agent embodied in the form of anti-DNA antibodies can be injected into a systemic blood circulation. In addition a modified agent which modifies the chemical composition and/or conformation and/or polymery and/or an association with proteins and/or lipids and/or ribonucleic acids of the blood extracellular DNA. Said modifying agent can be embodied in the form of a enzyme-ribonuclease. An extracellular ribonuclease of a Serratia Mercenses bacterium can be used in the form of said agents.
