Background: This randomized controlled trial assessed the safety and effects of twice-weekly weight training among recent breast cancer survivors. Outcomes included body size and biomarkers hypothesized to link exercise and breast cancer risk. Methods: A convenience sample of 85 recent survivors was randomized into immediate and delayed treatment groups. The immediate group trained from months 0 to 12; the delayed treatment group served as a no exercise parallel comparison group from months 0 to 6 and trained from months 7 to 12. Measures at baseline, 6 and 12 months included body weight, height, body fat, lean mass, body fat %, and waist circumference, as well as fasting glucose, insulin, insulin resistance, insulin-like growth factor-I (IGF-I), IGF-II, and IGF-binding protein-1, IGFBP-2, and IGFBP-3. Injury reporting was standardized. Results: The intervention resulted in significant increases in lean mass (0.88 versus 0.02 kg, P < 0.01), as well as significant decreases in body fat % (-1.15% versus 0.23%, P = 0.03) and IGF-II (-6.23 versus 28.28 ng/mL, P = 0.02) comparing immediate with delayed treatment from baseline to 6 months. Within-person changes experienced by delayed treatment group participants during training versus no training were similar. Only one participant experienced a study related injury that prevented continued participation. Conclusion: Twice-weekly weight training is a safe exercise program for recent breast cancer survivors that may result in increased muscle mass, as well as decreased body fat % and IGF-II. The implications of these results on cancer recurrence or survival may become more evident with longer exercise intervention trials among breast cancer survivors.