Background: Physical inactivity and being overweight are modifiable lifestyle risk factors that consistently have been associated with a higher risk of postmenopausal breast cancer in observational studies. One biologic hypothesis underlying this relationship may be via endogenous sex hormone levels. It is unclear if changes in dietary intake, physical activity, or both, are most effective in changing these hormone levels. Objective: This systematic review and meta-analysis examines the effect of reduced caloric dietary intake and/or increased exercise levels on breast cancer-related endogenous sex hormones. Methods: We conducted a systematic literature search in MEDLINE, Embase, and Cochrane's Central Register of Controlled Trials (CENTRAL) up to March 2017. Main outcome measures were breast cancer-related endogenous sex hormones. Randomized controlled trials (RCTs) reporting effects of reduced caloric intake and/or exercise interventions on endogenous sex hormones in healthy, physically inactive postmenopausal women were included. Studies including women using hormone therapy were excluded. The methodological quality of each study was assessed by the Cochrane's risk of bias tool. Results: From the 2599 articles retrieved, seven articles from six RCTs were included in this meta-analysis. These trials investigated 1588 healthy postmenopausal women with a mean age ranging from 58 to 61 years. A combined intervention of reduced caloric intake and exercise, with durations ranging from 16 to 52 weeks, compared with a control group (without an intervention to achieve weight loss) resulted in the largest beneficial effects on estrone treatment effect ratio (TER) = 0.90 (95% confidence interval (CI) = 0.83-0.97), total estradiol TER = 0.82 (0.75-0.90), free estradiol TER = 0.73 (0.66-0.81), free testosterone TER = 0.86 (0.79-0.93), and sex hormone biding globulin (SHBG) TER = 1.23 (1.15-1.31). A reduced caloric intake without an exercise intervention resulted in significant effects compared with control on total estradiol TER = 0.86 (0.77-0.95), free estradiol TER = 0.77 (0.69-0.84), free testosterone TER = 0.91 (0.84-0.98), and SHBG TER = 1.20 (1.06-1.36). Exercise without dietary change, versus control, resulted in borderline significant effects on androstenedione TER = 0.97 (0.94-1.00), total estradiol TER = 0. 97 (0.94-1.00), and free testosterone TER = 0. 0.97 (0.95-1.00). Conclusions and relevance: This meta-analysis of six RCTs demonstrated that there are beneficial effects of exercise, reduced caloric dietary intake or, preferably, a combination of exercise and diet on breast cancer-related endogenous sex hormones in physically inactive postmenopausal women. Keywords: Breast cancer; Caloric restriction; Exercise; Postmenopausal women; Prevention; Sex hormones; Weight loss.