The purpose of the study was to investigate whether exposure to 900MHz GSM wireless communication signals at whole-body SAR levels of 4.0, 1.33, 0.44 and 0 W/kg enhances mammary tumor development and growth induced by low-dose DMBA. Exposure started on the day after DMBA administration and lasted 4 h/day, 5 days/week for 26 weeks. There were no statistically significant differences in body weight between sham- and GSM microwave-exposed groups. No significant differences in overall mammary tumor incidence, latency to tumor onset, tumor multiplicity, or tumor size were observed between microwave- and sham-exposed groups. This study does not provide evidence that GSM microwave exposure promotes mammary tumor development in rats. In the present study there were significant differences between the cage controls and the experimental groups (sham and exposure). Body weight and mammary tumor (malignant plus benign) incidence in the cage control group were significantly higher than in the sham- and GSM microwave-exposed groups. The latency to the mammary tumor onset was significantly shorter in the cage control group than in the other groups.