SummaryA cross-regulation between two regulatory T cell (Treg) subsets [CD4 + CD25 + and invariant natural killer (NK) T -iNK T] has been described to be important for allograft tolerance induction. However, few studies have evaluated these cellular subsets in stable recipients as correlates of favourable clinical outcome after heart transplantation. Treg and iNK T cell levels were assayed by flow cytometry in peripheral blood samples from 44 heart transplant recipients at a 2-year interval in 38 patients, and related to clinical outcome. Multiparameter flow cytometry used CD4/CD25/CD127 labelling to best identify Treg, and a standard CD3/CD4/CD8/Va24/Vb11 labelling strategy to appreciate the proportions of iNK T cells. Both subtypes of potentially tolerogenic cells were found to be decreased in stable heart transplant recipients, with similar or further decreased levels after 2 years. Interestingly, the patient who presented with several rejection-suggesting incidents over this period displayed a greater than twofold increase of both cell subsets. These results suggest that CD4 + CD25 + CD127 low/neg Treg and iNK T cells could be involved in the local control of organ rejection, by modulating immune responses in situ, in clinically stable patients. The measurement of these cell subsets in peripheral blood could be useful for non-invasive monitoring of heart transplant recipients, especially in the growing context of tolerance-induction trials.