An 11-year-old boy with acute myeloblastic leukaemia in first remission received an allogeneic mismatched bone-marrow transplant (BMT) from his father in 1979; subsequent HLA typing showed that his haemopoietic system had been repopulated by the donor cells. In 1986 hypereosinophilic syndrome, secondary to a T-cell lymphocytic lymphoma, developed in the father, then aged 45 years. A full haematological remission was obtained by means of standard acute lymphoblastic leukaemia treatment. He then received melphalan, total body irradiation, and a BMT from his son. Graft-versus-host disease was transient in both patients, and father and son remain well and disease-free 20 months and 10 years, respectively, after BMT.