Allogeneic bone marrow transplantation (BMT) was first explored for the treatment of multiple myeloma (MM) in the mid 1980s. Since then, there has been a rapidly growing clinical demand for the use of this modality of treatment, so that the number of patients receiving worldwide transplants from HLA-identical siblings is actually estimated to be at least several hundreds. Although it is difficult to compare results between different centers because of differences in patient characteristics, selection criteria for transplantation and conditioning regimens, a certain number of conclusions have emerged from these experiences. There is evidence that allogeneic BMT performed in different phases of MM and with different conditioning regimens yields a high frequency of complete remissions (CR), in the range of 50% to 60%, and long-term survival and remission rates, both averaging approximately 20%. Although a toxic-related mortality rate of 40% has been consistently reported for many years, the outcome of patients in whom BMT was performed as consolidation of remission has recently improved. Prior responsiveness to conventional chemotherapy, also the presence of low tumor cell mass (both at diagnosis and before transplant) predicts for increased CR rate (up to 70% or more), as well as long-term survival and remission rates (both averaging approximately 50%). The continued relapse-free survival, up to and beyond ten years, reported for some of these patients provides strong evidence that allogeneic BMT has the potential ability to cure MM, probably as the result of an immunologic effect of the infused donor's marrow T lymphocytes against residual myeloma cells (graft-versus-myeloma).(ABSTRACT TRUNCATED AT 250 WORDS)