Improper T-cell reconstitution with its consequences, graft-vs-host disease (GvHD) and outbreak of viral infections, is the major cause of morbidity and mortality after hematopoietic stem cell transplantation (HSCT). To determine the factors affecting reconstitution of naive T-cells after non-myeloablative HSCT (NM-HSCT), the T-cell receptor excision circle (TREC) content was measured on a weekly basis in 24 transplanted patients with various malignant diseases. We analysed correlations of the results with the development of GvHD. In addition, in 11 chronic myeloid leukaemia (CML) patients, we correlated TREC and BCR-ABL transcript numbers. After HSCT, in most patients (22/24) TRECs became undetectable. In 12 patients, TRECs reappeared 3?4 months after HSCT, in 1 patient TRECs reappeared 5 months after HSCT, and in 11 patients TRECs remained negative for more than a year. All 11 patients who remained TREC-negative, developed acute GvHD grade 2?3, while only 6 out of 13 patients who recovered TRECs developed GvHD. We show that after non-myeloablative HSCT, thymopoiesis takes place and is affected by GvHD. Our results indicate that no recovery of TRECs after NM-HSCT (which most likely reflect the expansion of host-reactive co-transplanted mature T-cells) correlates with the onset of GvHD.