In order to evaluate the value of isovolumic relaxation time measurement for the diagnosis of moderate acute rejection episodes in cardiac allograft recipients a comparison was made with the histological results from the endomyocardial biopsy. A total of 202 isovolumic relaxation time measurements from 26 patients were compared to the biopsy results. The technique used to record isovolumic relaxation time was dual M-mode echocardiography. In addition a combined phonoechocardiography was used for 54 isovolumic relaxation time measurements from 17 patients. A good correlation was found between these two methods. When the biopsy results were normal the isovolumic relaxation time was 71.4 +/- 15.1 ms. When moderate acute rejection episodes were present isovolumic relaxation time decreased to 50.2 +/- 21.2 ms (p < 0.001). In spite of the close correlation detected at group level, there was a large variability of the measurements without accompanying changes in the biopsy specimen. At the same time a significant overlap was found between the measurements taken during rejection and non-rejection periods making it impossible to use these methods for clinical decision making. We conclude that isovolumic relaxation time measured with these methods is not a sufficiently sensitive parameter for the diagnosis of moderate acute rejection episodes in the individual patient, and in our experience, it is not a substitute for endomyocardial biopsy and can not be applied for clinical decision making.