Marked sensitivity of tumor tolerance to blood glucose level is demonstrated in a mouse model of human breast cancer. A theory is cited that explains the high association of hyperglycemia with malignancy, especially breast cancer, via glycemic modulation of cellular immunity. Three groups of BALB/C mice were injected with an aggressive mammary tumor and placed on three dietary regimens designed to produce three different glycemic levels. Mortalities 70 days after injection were 16 of 24 hyperglycemic mice, 8 of 24 normoglycemic, and 1 of 20 hypoglycemic (chi-square p less than .005). Taken together with other experiments and human data discussed briefly, this result suggests that glycemic modulation of tumor tolerance should be evaluated in human trials.