To determine if previous donor pregnancies influence the development of acute graft-versus-host disease (GVHD) we evaluated data from 136 patients with aplastic anaemia greater than 15 years of age and given marrow grafts from HLA-identical sibling donors. Of the 136 marrow donors, 30 were parous females (previous history of pregnancy), 30 were nulliparous females (no history of pregnancy or abortions), and 76 were males. The cumulative incidence of grade II-IV GVHD was 57%, 21% and 46% for patients with parous, nulliparous and male donors, respectively. A multivariate analysis of the data confirmed that the risk of grade II-IV acute GVHD was significantly increased among patients receiving marrow from parous females as compared to those from nulliparous females (relative risk = 2·5, P= 0·02). There was no statistically significant difference in the incidence of acute GVHD, however, between patients with parous donors and male donors (relative risk = 1·3, P= 0·26). Male patients given grafts from parous donors showed a higher incidence of acute GVHD (63%) than female patients (45%), though this difference was not statistically significant. The 5-year probability of survival was 47% for patients with parous donors, 68% for patients with nulliparous donors and 70% for those with male donors. We confirm that prior donor pregnancy represents an important factor in selecting marrow donors or designing clinical protocols for GVHD prophylaxis.