Background Chemokines play an essential role in regulating the infiltration of leukocytes into allografts in experimental models. Little is known of their expression or function after human cardiac transplantation. Methods and Results We analyzed 169 sequential human endomyocardial biopsies by immunocytochemistry for infiltration by CD3 ⁺ T cells and the expression of the chemokine receptors CCR1, CCR3, CCR5, and CXCR3. In both cross-sectional and longitudinal analyses, the expression of each of the chemokine receptors correlated with the degree of CD3 ⁺ T-cell infiltration. In particular, the expression of CXCR3 was temporally and spatially associated with CD3 ⁺ T-cell infiltrates and correlated with the histopathological diagnosis of acute rejection (OR, 11.73 and 4.05, respectively; P <0.001). Of 7 patients followed up longitudinally for 1 year, 4 with consecutive biopsies developed intimal thickening by intravascular ultrasound. In these patients, there was a trend for persistent expression of CD3- and CXCR3-expressing infiltrates in the later part of the first posttransplant year. The chemokines eotaxin, IP-10, lymphotactin, MCP-1, Mig, RANTES, and SDF-1 were examined in an additional 35 biopsies by RT-PCR. Eotaxin, lymphotactin, MCP-1, Mig, and SDF-1 were present in both normal and rejecting biopsies. However, the CXCR3 ligand IP-10, which was rarely expressed in normal biopsies, was markedly induced in acute rejection (OR, 19.43; P =0.01). Conclusions The presence of CXCR3 ⁺ T cells and the CXCR3 ligand IP-10 within endomyocardial biopsies is strongly associated with acute rejection. The CXCR3–IP-10 interaction warrants consideration as a therapeutic target in the management of cardiac allograft recipients.