Activin A is a dimeric protein that regulates endometrial functions by signaling at its receptors, namely type I (ActRI) and type II (ActRII). Nodal is an activin competitor that requires the coreceptor cripto to assemble its signaling pathway through ActRI and ActRII. In the current study, we evaluated the expression of activin A, ActRII, nodal, and cripto in eutopic and ectopic endometrium collected from women with ovarian endometrioma (n = 15) and in eutopic endometrium of healthy participants (n = 15). Eutopic endometrial samples were evaluated according to the stage of menstrual cycle. Total RNA was extracted from tissue homogenates and analyzed by real-time polymerase chain reaction (PCR). Activin A messenger RNA (mRNA) expression in eutopic endometrium of patients with endometriosis was significantly higher than in controls (P < .001) with a 10.2-fold and 7.3-fold increase in the proliferative and secretory phases, respectively. ActRII and nodal mRNA expression were found to be similar in patients with and without endometriosis, while cripto mRNA was markedly lower in eutopic (fold change = 0.03 at proliferative phase, P < .001) and ectopic endometrium (fold change = 0.14, P < .001) of women with endometriosis compared with eutopic endometrium from healthy controls. In conclusion, the altered endometrial expression of activin A and cripto during the menstrual cycle and the differences observed in the endometriotic tissue support the involvement of the activin system in endometrial changes of women with endometriosis.