During the rejection process of cardiac allografts, the expression of HLA antigens increases on various graft tissues, ie, the myocardium and the interstitial structures. However, in this type of transplant there is a paucity of knowledge about HLA expression on recipient cells, such as peripheral blood mononuclear cells. In the present study expression of HLA class I and class II antigens was monitored on peripheral blood lymphocytes prior to and during a 12-month follow-up, using flow cytometry. In our series, the frequency of acute rejection episodes was greater from the fourth to the ninth month after transplantation, coinciding with a reduction in cyclosporine blood levels. At the same time, expression of HLA class I and class II antigens significantly increased among recipients suffering from more severe acute rejection episodes compared with those showing acceptance of their grafts (P < .01). In conclusion, acute rejection episodes in cardiac transplantation were associated with up-regulation of HLA molecules on recipient peripheral blood cells. Monitoring the expression of HLA molecules on peripheral blood lymphocytes may represent an easy, noninvasive practice to individualize immunosuppressive therapy.