The diagnosis of acute rejection remains a key issue in the management of the heart transplant recipient. Myocardial biopsy for tissue examination is the basic step for screening and diagnosis of acute rejection. Although endomyocardial biopsy is reliable, it is an inefficient approach to screening after transplantation and yielded only a 14% rate of positive results in the author's experience, from 1983 to 1990, of 568 biopsies. A reliable noninvasive method for screening acute rejection is therefore needed. Numerous noninvasive methods have been studied to monitor the systemic immune process against the allograft or to evaluate the effect of rejection on graft function and status. For 13 methods of evaluating immune process against the allograft the sensitivity and specificity ranged from 13% to 95% and 19% to 94% respectively. For nine methods of evaluating allograft function, sensitivity and specificity ranged from 60% to 93% and 65% to 97% respectively. Overall, methods monitoring allograft function and status have better results in predicting acute rejection. Nevertheless, the author estimated that 15 episodes of acute rejection would have been missed by these monitoring methods in his group of patients.