Two cases of spontaneous tumor regression (STR) occurring in a patient with non Hodgkin lymphoma and in another patient with squamous carcinoma of the lung are presented. Both cases fulfill the criteria of STR defined by Everson and Cole. Recent results obtained in basic and clinical studies have indicated that immunological mechanisms could play an important role in STR. The mediator effects more frequently referred are: 1) generation of antineoplastic cytotoxic cells; 2) production of immunoregulatory cytokines by lymphocytes and monocytes, and 3) possible cross reaction between tumor and bacterial antigens. These mechanisms of action are discussed in relation to the presented cases.