Heart transplantation is, today, an accepted and recommended modality in the management of selected patients suffering from terminal heart disease.1 However, acute rejection and infection remain the major complications of this operation. Serial endomyocardial biopsy (EB), considered as the standard for diagnosis of cardiac rejection, is an invasive and delicate operation, not free of complications, even when done by skilled personnel in specialized centers.2,3 The object of this study was to compare and correlate between radionuclide ventriculography (RNV) and the histologic findings of EB. Furthermore, to validate the use of nuclear cardiology techniques that allow noninvasive, reliable, and rapid quantitation of ventricular function and myocardial perfusion for the diagnosis and management of rejection in patients with heart transplants. Radionuclide studies of left ventricular function were performed in 3 heterotopic heart transplant patients (HHT) with long term survival and early after the operation in 5 patients with HHT, 12 orthotopic heart transplants (OHT) and in 2 heart and lung transplants (HLT). Simultaneous EBs were performed in the early posttransplant patients and a histologic score for acute rejection was obtained. First pass (FP) and multigated equilibrium blood pool ventriculography, using the in vivo 99mTc-labelling of RBCs was used to measure left ventricular volumes (LVV) such as stroke volume (SV), end-diastolic volume (EDV), end-systolic volume (ESV), and both global and regional ejection fraction (EF, REF).4,5 The histological grading of acute rejection was classified into four groups: (1) no rejection, (2) mild rejection, (3) moderate rejection, and (4) severe rejection.6 The median of each LVV parameter was calculated and correlated with the EB using a nonparametric one way analysis of variance. A percentage change of LVVs was used rather than the difference of the calculated LVVs.7 During moderate acute rejection, SV had the highest correlation in P<0.004, followed by the EDV (P<0.05), and finally ESV (P<0.02). During severe acute rejection the correlation was SV (P<0.0008), EDV (P<0.001), and ESV (P<0.006). Myocardial perfusion scintigraphy using 201T1 was performed in the HHT patients, although, at this stage we have not attempted a correlation with the histologic findings. In one patient with long term survival OHT, increased 131I-metaiodobenzylguanidine (MIBG) myocardial uptake was evident during a rejection episode. © 1988 Grune & Stratton, Inc.