Insulin may affect breast cancer (BC) risk and prognosis. Exercise reduces insulin in obese BC survivors. We designed a randomised controlled trial to test the effect of an aerobic exercise intervention (AEI) on insulin parameters and body composition in non-obese BC women without insulin resistance. Thirty-eight BC women were randomised into an intervention group (IG = 18) or control group (CG = 20). IG participated in a structured AEI for 3 months, while CG received only the Word Cancer Research Fund/American Institute Cancer Research (WCRF/AICR) recommendation to be physically active. Fasting insulin, homeostasis model assessment of insulin resistance (HOMA-IR) index, metabolic parameters and body composition were collected at baseline and after the AEI. IG reduced insulin and HOMA-IR index by 15% and 14%, while CG increased these parameters (+12% and +16%). Insulin changed differently over time in the two randomised groups (pinteraction = .04). The between-group differences in the change of insulin (IG = -1.2 μU/ml versus CG = +0.8 μU/ml) and HOMA-IR index (IG = -0.26 versus CG = +0.25) were respectively significant (p = .04) and non-significant (p = .06). IG significantly improved lower limb muscle mass in comparison with CG (p = .03). A structured AEI may improve insulin, HOMA-IR index and body composition in non-obese BC survivors without insulin resistance.