OBJECTIVE Detection of acute allograft rejection in heart transplant recipients by noninvasive methods is a challenge in the management of these patients. In this study, the usefulness of a new highly sensitive method for the measurement of troponin T is evaluated. METHODS We designed a case-crossover study, in which each patient served as his or her own control, by selecting samples from treated acute rejection episodes (29 cases) and samples obtained immediately before and/or after rejection (38 controls). The highly sensitive troponin T was measured by a new pre-commercial test (Elecsys Troponin T HS). RESULTS In all samples, highly sensitive troponin T was detectable, with a median of 0.068 ng/L (IQR, 0.030-0.300 ng/L). The levels correlated with right atrial pressure (r=0.37; P=.002), N-terminal pro-brain natriuretic peptide concentration (r=0.67; P<.001), and time since transplantation (r=-0.81; P<.001). The highly sensitive troponin T concentrations were higher in patients with rejection (0.155 ng/mL vs 0.047 ng/mL; P=.006). In the receiver operating characteristic analysis, the area under the curve was 0.67 (95% confidence interval, 0.53-0.77) and the best cutoff was 0.035 ng/mL, which was associated with rejection (odds ratio=3.7; 95% confidence interval, 1.2-11.9; P=.02). By restricting the analysis to the first 2 months, the area under the curve increased to 0.86 (95% confidence interval 0.66-0.97), with an optimal cutoff of 1.10 ng/mL (S=58% [28%-85%]; E=100% [74%-100%]). CONCLUSIONS Troponin T was detectable in all samples when a new highly sensitive assay was used, and at higher concentrations in the presence of acute rejection; however, the usefulness of this test in patient management is limited to support for clinical or histological suspicion of rejection, especially in the early post-transplant period.