We have evaluated post-transplant serum levels of prolactin with respect to source of donated bone marrow (BM) and to the occurrence of either acute and/or chronic graft-versus-host disease (GVHD). Forty adult patients underwent allogeneic (n = 35), autologous (n = 4) or syngeneic (n = 1) bone marrow transplantation for haematologic malignancy (n = 32) or aplastic anaemia (n = 8), respectively. Serum prolactin levels measured within 100 days post-transplant were related to patients' sex but otherwise proved unrelated to the occurrence or severity of GVHD and to the source of the BM graft (allogeneic, autologous, syngeneic). Beyond day 100 post-graft, however, serum prolactin levels proved significantly elevated in allogeneic recipients exhibiting chronic GVHD (p = 0.0004) and were unrelated to the patients' sex. In this group of patients, serum prolactin levels were not related to serum cyclosporin levels. In allogeneic recipients exhibiting no GVHD, serum prolactin levels were positively correlated with serum cyclosporin levels (p < 0.05). These data show that serum prolactin levels are significantly elevated beyond day 100 post-graft in recipients exhibiting chronic GVHD. Prolactin, a hormone recently shown also to be released by mononuclear leucocytes and to be involved in lymphocyte activation plays a hitherto unrecognized role in the pathogenesis of GVHD in humans.