Chimerism can be monitored after HLA-matched allogeneic bone marrow transplantation (BMT) or allogeneic peripheral blood stem cell transplantation (PBSCT) by detecting polymorphisms in short tandem repeats (STR). The purpose of our study was to document early complete chimerism in BMT and PBSCT recipients using STR, and to determine whether the initial WBC recovery correlated with the days required to attain complete chimerism. A total of 5 patients (2 PBSCT and 3 BMT) were followed by STR after transplantation. Peripheral blood obtained prior to transplantation was used to determine the 2 most informative STR probes for each donor/recipient pair. STR were amplified by polymerase chain reaction (PCR) with 8 commercial probes, and PCR products were visualized with silver staining. Peripheral blood was evaluated daily post-transplantation for WBC counts and to identify the presence of mixed or full chimerism by STR. The sensitivity of the STR technique varied from 0.05 to 1%, depending on the probe. Full chimerism was documented between day 9 and 14 in PBSCT recipients and on day 14 and 16 in BMT recipients. The initial rise in WBC occurred within 3 days of the onset of full chimerism, indicating that full chimerism is a more sensitive indicator of early engraftment. Periodic recipient monitoring using STR after complete chimerism identifies those patients who revert to mixed chimeras. The STR method may be useful in future studies to determine the significance of early engraftment and the clinical implications of sustained complete chimerism or mixed chimerism.