To study the relationship between graft rejection and microchimerism with and without donor bone marrow infusion in recipients of kidney, liver, heart and lung transplants. 
Selection of manuscripts. Thirty‐seven manuscripts presenting clinical data on microchimerism and rejection, published between 1991 and 1997, were identified. Of these, 16 were excluded due to duplication or insufficient data. Inclusion criteria were data on microchimerism, bone marrow infusion and rejection episodes. 
 A mixed effect logistic model was used to test for homogeneity of transplant centers. The centers were found to be homogeneous for rejection rates controlling for microchimerism and bone marrow infusion. Using rejection episodes at 3, 6, and 12 months post‐transplant as the outcome, we evaluated logistic regression models to derive odds ratios for rejection with microchimerism and with bone marrow infusion for each organ. 
 Microchimerism was generally associated with a higher incidence of acute rejection for heart, lung, and kidney transplants and a lower incidence for liver transplants, especially at 12 months and above. Bone marrow infusion decreased the risk of acute rejection for heart transplants and increased the risk for lung and, to a lesser extent, for liver transplants. No consistent effect was seen in kidney transplants. At 12 months and longer, microchimerism was associated with a decreased incidence of chronic rejection in recipients of lung transplants, but there were insufficient data to determine this outcome for other organs. 
 (i) Microchimerism was detected in the majority of patients. (ii) The effect of microchimerism and bone marrow infusion on rejection episodes varied with the organ and, for a given organ, it was time‐dependent. (iii) These findings demonstrate the need for more extensive studies on microchimerism and donor‐specific hyporesponsiveness.