Allogeneic stem cell transplantation is potentially curative, but associated with post-transplantation complications, including cytomegalovirus (CMV) infections. An effective immune response requires T cells recognizing CMV epitopes via their T cell receptors (TCRs). Little is known about the TCR repertoire, in particular the TCR-α repertoire and its clinical relevance in patients following stem cell transplantation. Using next-generation sequencing we examined the TCR-α repertoire of CD8+ T cells and CMV-specific CD8+ T cells in four patients. Additionally, we performed single-cell TCR-αβ sequencing of CMV-specific CD8+ T cells. The TCR-α composition of human leucocyte antigen (HLA)-A*0201 CMVpp65– and CMVIE-specific T cells was oligoclonal and defined by few dominant clonotypes. Frequencies of single clonotypes reached up to 11% of all CD8+ T cells and half of the total CD8+ T cell repertoire was dominated by few CMV-reactive clonotypes. Some TCR-α clonotypes were shared between patients. Gene expression of the circulating CMV-specific CD8+ T cells was consistent with chronically activated effector memory T cells. The CD8+ T cell response to CMV reactivation resulted in an expansion of a few TCR-α clonotypes to dominate the CD8+ repertoires. These results warrant further larger studies to define the ability of oligoclonally expanded T cell clones to achieve an effective anti-viral T cell response in this setting.