Background— Perinatal and pediatric autopsies have declined worldwide in the past decade. We compared the diagnostic accuracy of postmortem, cardiovascular magnetic resonance (CMR) imaging with conventional autopsy and histopathology assessment in fetuses and children. Methods and Results— We performed postmortem magnetic resonance imaging in 400 fetuses and children, using a 1.5-T Siemens Avanto magnetic resonance scanner before conventional autopsy. A pediatric CMR imager reported the CMR images, masked to autopsy information. The pathologists were masked to the information from CMR images. The institutional research ethics committee approved the study, and parental consent was obtained. Assuming a diagnostic accuracy of 50%, 400 cases were required for a 5% precision of estimate. Three cases were excluded from analysis, 2 with no conventional autopsy performed and 1 with insufficient CMR sequences performed. Thirty-eight CMR data sets were nondiagnostic (37 in fetuses ≤24 weeks; 1 in a fetus >24 weeks). In the remaining 359 cases, 44 cardiac abnormalities were noted at autopsy. Overall sensitivity and specificity (95% confidence interval) of CMR was 72.7% (58.2–83.7%) and 96.2% (93.5–97.8%) for detecting any cardiac pathology, with positive and negative predictive values of 72.7% (58.2–83.7%) and 96.2% (93.5–97.8%), respectively. Higher sensitivity of 92.6% (76.6–97.9%), specificity of 99.1% (97.4–99.7%), positive predictive value of 89.3% (72.8–96.3%), and negative predictive value of 99.4% (97.8–99.8%) were seen for major structural heart disease. Conclusions— Postmortem CMR imaging may be a useful alternative to conventional cardiac autopsy in fetuses and children for detecting cardiac abnormalities. Clinical Trial Registration— URL: http://www.clinicaltrials.gov . Unique identifier: NCT01417962.