In a prospective protocol for noninvasive diagnosis and follow-up of acute heart rejection 162 examinations were performed in 36 patients who underwent heart transplantation. The follow-up period ranged from 15 days to 44 months. The protocol comprised multiple gated acquisition ventriculography with albumin labeled with 99mTc (740 MBq), acquired using a forward/backward by thirds framing mode, 32 frames/cycle, and 10 million total counts. Parameters of left ventricular diastolic function were analyzed. Antimyosin antibody labeled with indium 111 (74 MBq) was injected, and myocardium/lung uptake ratios were obtained at 48 hours in counts per pixel. Endomyocardial biopsy was performed in all patients within 48 hours. The results were evaluated by comparison of mean values of each parameter and global and individual correlation analysis in relation to the presence or absence of rejection and treatable (moderate or severe) or nontreatable (mild or absent) rejection. Antimyosin and diastolic function parameters showed significant differences (p less than 0.001) between patients with and without rejection and between patients with treatable and nontreatable rejection. Global correlation with biopsy existed (p less than 0.05) for antimyosin (r = 0.75), average filling rate (r = 0.61), and peak filling rate (r = 0.56). Individual correlation exhibited significance in all patients only for antimyosin (r = 0.78 to 0.98). In eight patients average filling rate also showed significant correlation (r = 0.65 to 0.88). In conclusion, these results provide a noninvasive diagnosis of cardiac allograft rejection episodes and allow an accurate selection between treatable and nontreatable rejection. Individual patient follow-up is possible with antimyosin. The study of diastolic function is also useful in this setting.