INTRODUCTION Accumulating evidence indicates that interleukin (IL)-34 participates in T-cell homeostasis and tolerance due to the ability of IL-34 to trigger apoptosis of Th1, Th17, and Tc1 cells, but spare Th2 cells and Treg. In addition, IL-34 exerts anti-inflammatory effects by impairing leukocyte adhesion and transendothelial migration, and reducing the secretion of proinflammatory cytokines. The aim of our study was to investigate the time course of serum levels of IL-34 during hepatic allograft rejection. METHODS Serum levels of IL-34 were determined in 20 healthy subjects and 45 hepatic transplant recipients. These patients were divided into 2 groups: group I was composed of 15 patients with acute rejection, and group II was composed of 30 patients without acute rejection. Samples were collected on days 1 and 7 after liver transplantation and on the day of liver biopsy. RESULTS The concentrations of IL-34 were higher in the rejection group vs nonrejection group during the entire postoperative period. The whole transplant group, including those with stable graft function, showed higher IL-34 serum levels than the controls at all times after liver transplantation. CONCLUSIONS Our preliminary results could be related to the recent finding that IL-34 may play an immune-suppressive role in liver transplantation. In our case, although we must be cautious with serum data, increased IL-34 would help to control alloresponse during rejection and protect from graft lost.