Activin A and follistatin are growth factors produced by several organs, comprising the endometrium, where they modulate cell and tissue differentiation. In this study, the authors tested whether activin A and follistatin are measurable in menstrual blood and whether their concentrations change in women with dysfunctional uterine bleeding (DUB). The authors evaluated healthy women with regular menstrual cycles (n = 15) and women with DUB (n = 12). Activin A and follistatin were measured in both menstrual and peripheral blood samples using highly sensitive enzyme immunoassays, whereas their respective mRNAs were quantified by real-time polymerase chain reaction in endometrial samples collected during the perimenstrual period. Activin A concentrations were 4-fold higher in menstrual than in peripheral serum of healthy women (mean +/- SE, 4.24 +/- 0.18 vs 1.00 +/- 0.15 ng/mL, P < .001) and were significantly lower in women with DUB compared to healthy subjects (P < .001). Follistatin concentration was 8-fold higher in menstrual than in peripheral serum of healthy women (3.94 +/- 0.49 vs 0.49 +/- 0.04 ng/mL, P < .001) and was significantly lower in the menstrual serum of women with DUB compared to controls (P < .001). There was no correlation between menstrual and peripheral serum concentrations of both proteins. The endometrial expression of activin A and follistatin mRNA was lower in women with DUB compared to controls (P < .05). Both activin A and follistatin are measurable in high concentrations in human menstrual blood and are relatively lower in women with DUB. The quantitative assessment of activin A and follistatin in menstrual serum might be a putative clinical marker of endometrial function.