Regulatory T lymphocytes (Treg) suppress activation of the immune system and prevent pathological autoreactivity, giving them a relevant role in transplantation. In this study, we compared the proportion of Treg in a group of kidney transplant recipients with those in a control group. We used flow cytometry and labeling with monoclonal CD4, CD25, and FoxP3 antibodies to analyze the percentage of Treg lymphocytes in peripheral blood in a group of 68 patients at more than 12 years since transplantation and in 16 untransplanted healthy controls. In addition to the laboratory determinations, we analyzed the effect of some clinical parameters on the percentage of Treg in the transplanted group with a previous history of hepatitis C virus infection and undergoing immunosuppressive treatment. The percentage of Treg levels observed in the transplanted group was significantly lower than that in the control group (1.53% vs 2.89% CD4+ T cells; P = .0022). The percentage of Treg cells was significantly lower among patients treated with mycophenolate mofetil (1.12%) than other drug combinations. We also compared the percentage of Treg between transplant recipients treated with immunosuppressive monotherapy and those treated with combined immunosuppression, observing a higher percentage among patients with monotherapy.