Approaches to the measurement of lymphohematopoietic chimerism have evolved from laboratory research to important clinical tools. However, there has been no logical, consistent, and uniform set of recommendations for the measurement of chimerism in clinical transplantation. The National Marrow Donor Program and the International Bone Marrow Transplant Registry (IBMTR) sponsored a workshop to discuss the use of chimerism analysis after allogeneic transplantation. The workshop was organized in an effort to make reasonable recommendations regarding laboratory techniques, the types of specimens to be studied, and the frequency of analysis. The panel recommended the following guidelines: 1. Chimerism analysis should use sensitive, informative techniques. At present, short tandem repeats (STR) or variable number tandem repeats (VNTR) analysis is the approach most likely to give reproducible informative data. 2. Peripheral blood cells are generally more useful than bone marrow cells for chimerism analysis. 3. Lineage-specific chimerism should be considered the assay of choice in the setting of nonmyeloablative and reduced-intensity conditioning. 4. The use of T-cell depletion, nonmyeloablative or reduced-intensity conditioning, or novel graft-versus-host disease (GVHD) prophylactic regimens warrants chimerism analysis at 1, 3, 6, and 12 months, because interventions such as donor lymphocyte infusions may depend on chimerism status. 5. In nonmyeloablative transplantation, the early patterns of chimerism may predict either GVHD or graft loss. Therefore, more frequent (every 2-4 weeks) peripheral blood analysis may be warranted. 6. For nonmalignant disorders, chimerism generally should be measured 1, 2, and 3 months after transplantation. Interventions to enhance donor engraftment must be considered on a disease-specific basis in relation to concurrent GVHD and, ultimately, clinical rationale.