The aim of this study was to test the hypothesis that echocardiographic strain imaging, by tracking subtle alterations in myocardial function, and cardiac magnetic resonance T1 mapping, by quantifying tissue properties, are useful and complement each other to detect acute cellular rejection in heart transplant recipients. Noninvasive alternatives to endomyocardial biopsy are highly desirable to monitor acute cellular rejection. Surveillance endomyocardial biopsies, catheterizations, and echocardiograms performed serially according to institutional protocol since transplantation were retrospectively reviewed. Sixteen-segment global longitudinal strain (GLS) and circumferential strain were measured before, during, and after the first rejection and at 2 time points for patients without rejection using Velocity Vector Imaging for the first part of the study. The second part, with cardiac magnetic resonance added to the protocol, served to validate previously derived strain cutoffs, examine the progression of strain over time, and to determine the accuracy of strain and T1 measurements to define acute cellular rejection. All tests were performed within 48 h. Median time to first rejection (16 grade 1 rejection, 15 grade ≥2 rejection) was 3 months (interquartile range: 3 to 36 months) in 49 patients. GLS and global circumferential strain worsened significantly during grade 1 rejection and ≥2 rejection and were independent predictors of any rejection. In the second part of the study, T1 time ≥1,090 ms, extracellular volume ≥32%, GLS >-14%, and global circumferential strain ≥-24% had 100% sensitivity and 100% negative predictive value to define grade ≥2 rejection with 70%, 63%, 55%, and 35% positive predictive values, respectively. The combination of GLS >-16% and T1 time ≥1,060 ms defined grade 1 rejection with 91% sensitivity and 92% negative predictive value. After successful treatment, T1 times decreased significantly. T1 mapping and echocardiographic GLS can serve to guide endomyocardial biopsy selectively.