This study was the second co-carcinogenic confirmation study utilizing the DMBA-induced mammary tumor model (Yu et al (2006)). Five groups of 100 Sprague-Dawley rats were exposed to 902-MHz GSM-type Signals (4 ,1.33, 0.46 W/kg, sham and control) after orally administering DMBA. Statistically significant differences between RF-exposed groups and the sham-exposed group were that all RF-exposed groups had, at various times, significantly more palpable tissue masses. There were fewer animals with benign neoplasms, but more with malignant tumours in the high-dose group. In addition, there were more adenocarcinomas in the low-dose group, more malignant neoplasms in the low- and high-dose groups, more animals with adenocarcinomas in the high-dose group, and fewer animals with fibroadenomas in the low- and mid-dose groups. The cage-control group had, when compared with the sham-exposed group, statistically significantly more palpable tissue masses, more benign and also more malignant neoplasms. The cage-control group had in most aspects the highest incidence and malignancy of neoplasms among all groups. No clear conclusions could be drawn from these findings.