CONTEXT Regeneration of the endometrium in each menstrual cycle is required for reproduction. Endogenous endometrial stem cells reside in the basalis layer and serve as a source of cells that differentiate to form the endometrium. Bone marrow-derived cells have been shown to take on functions outside the hematopoietic system. OBJECTIVE To investigate the possibility that cells of extrauterine origin could repopulate the endometrium. DESIGN, SETTING, AND PATIENTS Endometrium from 4 HLA-mismatched bone marrow transplant recipients (1998-2002) was evaluated for donor HLA expression. Each recipient had a bone marrow donor with an HLA type that enabled determination of the origin of any cell. Endometrial biopsies also were obtained from 4 healthy control women. MAIN OUTCOME MEASURE HLA type was determined by immunohistochemistry and by reverse transcription-polymerase chain reaction. RESULTS Donor-derived endometrial cells were detected in endometrial biopsy samples from all bone marrow recipients and accounted for 0.2% to 48% of epithelial cells and 0.3% to 52% of stromal cells. None of the controls demonstrated HLA mismatch in endometrial samples. CONCLUSION These findings demonstrate that endometrial cells can originate from donor-derived bone marrow cells and suggest that nonuterine stem cells contribute to the regeneration of endometrial tissue.