The present study was designed to determine the effects of physical training on the development of cancer induced by the injection of Ehrlich tumor cells in mice. Male Swiss mice were subjected to a swim training protocol (5 days per week for 6 weeks; 1 hour at 50% of maximal capacity - trained groups) or remained sedentary in their cages (sedentary groups). Inoculation of Ehrlich tumor cells was performed at the end of the 4^th week and the animals were sacrificed after 6 weeks of training. Heart and solid tumor weight were recorded and tumor volumes were calculated. Portions of the tumors were used for the evaluation of macrophages and neutrophil accumulation or fixed in neutral 10% buffered formalin for histological analysis. Tumor volume and weight were respectively approximately 270% and 280% greater in the sedentary group than in the trained mice. Macrophage infiltration in the tumor tissue was significantly lower in trained mice (0.65 ± 0.16 vs. 1.78 ± 0.43 macrophages x10³ in the sedentary group). Moreover, neutrophil accumulation in the tumor was slightly reduced after exercise training and the amount of tumor cells was reduced in trained mice. Exercise capacity was substantially increased in trained mice, as determined by a 440% increase in the exercise time at 50% of maximal capacity. In summary, swim training retarded the development of Ehrlich tumors in mice, accompanied by a reduction in macrophage infiltration and neutrophil accumulation. These findings provide conceptual support for clinical observations that controlled physical activities may be a therapeutically important approach to preventing cancer progression and may improve the outcome of cancer treatment.