Research carried out over past two decades revealed a number of substances present in plantborne foods with cancer preventive properties, antioxidative activity in particular. It has been also long recognized that toxic side effects of cancer chemotherapy are associated with the ability of anticancer drugs to stimulate oxidative stress in the human organism. Therefore, a number of attempts have been undertaken to combine cancer treatment with the application of natural antioxidants to improve patients' quality of life. The substances tested include antioxidant vitamins, some isolated polyphenols, as well as plant extracts e.g. from spinach or grape seeds. This approach, though based on edible food components, still is rather pharmacological than dietary. In contrast, our research represents a purely nutritional approach and is aimed at finding out whether special design of patients' diet, i.e. enriched in a food item with high antioxidative potential, can influence the final outcome of cancer therapy. The experiments were carried using tumor bearing mice fed with red beet juice ad libitum instead of water and treated with doxorubicin, an antitumor drug whose toxicity, especially cardiotoxicity, is known to result from massive induction of reactive oxygen species. Prior to experiments in vivo, we demonstarated in cultured cells that red beet juice does not interfere with doxorubicin activity. Several experimental schemes were tested with juice given to mice before or/and after treatment of two types of tumors: melanoma B16 resistant and leukaemia L1210 sensitive to doxorubicin. In the first case, we observed slight improvement of therapeutic affect of doxorubicin in mice given red beet juice 7 days before and 7 days after therapy, however it was not statistically significant. Interestingly, this improvement occurred despite appparent stimulation of tumor growth in mice drinking juice. In general, in all mice groups tested, those drinking red beet juice were more lively, displayed better appetite as if they tolerated better both the disease and chemotherapy. In leukaemia bearing mice, at least in some experiments, the significant increase (around 30% over the mice given doxorubicin alone) of lifespan was observed for animals fed with red beet juice 7 days before and 7 days after therapy and also their body mass decreased less. All our observations, including those escaping quantification, suggest that appropriate dietary intervention may considerably augment cancer chemotherapy and improve patients' quality of life. In separate series of experiments, we collected serum, leukocytes and hearts from animals treated with doxorubicin in or without conjunction with red beet juice application. These samples are currently processed to determine such markers of oxidative stress as lipid peroxidation, oxidative damage to proteins, genotoxicity in leukocytes and heart tissue.