Localised Kaposi's sarcoma (KS) was diagnosed 240 days after allogeneic bone marrow transplantation (BMT) in a severely immunosuppressed HIV negative patient with genetic predisposition. The tumour was of host origin, based on PCR amplification of DNA minisatellites. Treatment with radiotherapy prompted almost complete regression of the lesions. However the patient subsequently died with relapsed acute myelogenous leukaemia. Contrary to the incidence observed after organ allografts, the advent of KS appears to be exceptional after BMT, since only one case has been reported to date, following reinfusion of autologous marrow.