Haematopoietic stem cell transplantation (HSCT) is performed for treatment of a broad spectrum of illnesses. Reconstitution of an intact immune system is crucial after transplantation to avoid infectious complications, and above all, the establishment of T cell receptor (TCR) diversity is the most important goal in the procedure. Until recently, little has been known of the mechanism of T cell reconstitution in the very early period after HSCT. In this study, we analysed TCR repertoires sequentially in four patients with severe combined immunodeficiency (SCID) before and after HSCT. In all patients, the TCR repertoires were extremely abnormal before HSCT, whereas after transplantation there was progressive improvement in TCR diversity, based on analysis of the TCR Vbeta repertoire and CDR3 size distributions. Somewhat unexpectedly, there was a significant but transient expansion of TCR diversity 1 month after transplantation in all cases. Clonotypic analysis of TCRs performed in one case showed that many T cell clones shared identical CDR3 sequences at 1 month and that the shared fraction decreased progressively. These results indicate that early expansion of TCR diversity may reflect transient expansion of pre-existing mature T cells from the donor blood, independent of de novo T cell maturation through the thymus.