Introduction: Exercise training, an adjuvant therapy has a prophylactic effect on improvement of breast cancer. The purpose of this study was investigating the effect of endurance training on serum IL-8 concentration and tumor volume in breast cancer bearing mice. Materials and Methods: Forty female BALB/c mice (4 to 5 week years old, average 17 g weight) were randomly assigned into four groups: 1) exercise-tumor-exercise (ETE), 2) exercise-tumor-rest (ETR), 3) rest-tumor-exercise (RTE) and 4) rest-tumor-rest (RTR). For tumorigenesis, MC4-L2 cell lines were used. Endurance training was performed for 8 weeks before inducing tumor in which training was initiated with 50% VO₂max and reached 75% VO₂max in the 8th week. Endurance training was performed with the intensity of 50 to 65% VO₂max for 6 weeks after tumor induction. After training, serum IL-8 concentration was measured by ELISA. Data were analyzed by one-way ANOVA and Pearson correlation. Results: There were significant differences in serum IL-8 concentration and tumor volume among the groups (P=0.0001). Significant differences were between training groups after tumor induction with other groups. In addition, significant correlations were observed between serum IL-8 concentration and tumor volume among the groups (P>0.05). Conclusions: Endurance training with moderate intensity reduces tumor volume. IL-8 is involved in angiogenesis and tumor growth and the decreased level of IL-8 is one reason for the reduced tumor volume. Due to reduction of the level of this cytokine in groups doing exercise after malignancy, endurance training in addition to a prophylactic role is presumed to have adjuvant therapeutic effects on estrogen-receptor dependent tumors.