Our previous studies indicate donor antigen-specific hyporeactivity is a useful marker for identifying solid organ transplant recipients at low risk for immunological complications; the hyporeactive subgroup experiences a lower incidence of chronic rejection. One purpose of the current study was to determine whether hyporeactivity could be identified in pediatric kidney recipients and whether it correlated with improved graft outcome. Of 18 pediatric kidney recipients tested, 6 (33%) had developed donor antigen-specific hyporeactivity. All 18 experienced good graft outcome. Second, we determined whether donor antigen-specific hyporeactivity correlates with peripheral blood microchimerism and outcome in adult kidney recipients. Our previous studies of lung recipients demonstrated development of obliterative bronchiolitis in recipients with microchimerism who remain responsive, but not in recipients who had become hyporesponsive to donor antigen. Preliminary results in our current study of 23 adult kidney recipients indicate microchimerism for 6 (26%): 4 hyporesponsive and 2 responsive to donor antigen. Microchimerism was not detected for 17 recipients: 6 hyporesponsive and 11 responsive to donor antigen. One hyporesponsive/chimeric patient and 4 recipients negative for both parameters have been diagnosed with biopsy-proven chronic rejection. In summary, both hyporeactivity and chimerism are found at a higher frequency in lung than kidney recipients. Unlike lung recipients, not all hyporesponsive kidney recipients had peripheral blood chimerism. Additional numbers are needed to determine if microchimerism correlates with donor antigen-specific hyporeactivity or graft outcome.