Chronic lymphocytic leukemia (CLL) in a patient is treated by administering to the patient oxidatively stressed CLL cells. The CLL cells are oxidatively stressed extracorporeally, e.g. by subjection to oxygen/ozone mixtures, and preferably are simultaneously subjected to other stressors such as UV light. Preferably also, the CLL cells are autologous, and are contained in an aliquot of the patient's blood at the time of subjection to stressing.