Syngeneic graft-versus-host disease (sGVHD) has been described after hematopoietic cell transplantation (HCT) but remains poorly defined. We retrospectively reviewed adult syngeneic HCTs at our center (1980-2002) for sGVHD to investigate incidence, morbidity, and risk factors with a primary focus on parity. Among 119 transplantations, there were 21 cases of biopsy-proven sGVHD. The cumulative incidence was 18%, with multiorgan involvement in 6 cases and 1 death. sGVHD was more frequent when the donor was parous (32%) than nulliparous (9%) or male (13%; P =.03) and when the recipient was parous (31%) than nulliparous (7%) or male (13%; P =.02). Other univariable risk factors included older age (P <.01), busulfan/melphalan/thiotepa conditioning (P <.01), interleukin-2 (P =.02), HLA-A26 (P =.03), and more recent transplantation year (P <.01). Overall, risk factors were similar to those described in GVHD. Although an independent effect of parity could not be completely separated from other factors, donor and recipient pregnancy history merits further investigation.