Left ventricular function was evaluated in nine patients during biopsy proven acute rejection of heart allografts using computer-assisted analysis of echocardiograms. The results were compared with data obtained during their non-rejection state and with data obtained from 10 normal subjects. Left ventricular dimensions and parameters of diastolic and systolic function of non-rejecting hearts were similar to those of normals. Acute rejection was associated with increased left ventricular mural thickness and mass, and abnormal diastolic function characterized by a prolongation of the rapid filling period from 155 msec +/- 14 msec to 183 msec +/- 16 msec (p less than 0.05, mean +/- SE), and decreased normalized peak rates of left ventricular lengthening (from 5.3 sec-1 +/- 0.8 sec-1 to 3.9 sec-1 +/- 0.4 sec-1, less than 0.05) and posterior wall thinning (from 7.4 sec-1 +/- 1.0 sec-1 to 3.8 sec-1 +/- 0.6 sec-1, p less than 0.05). Parameters of systolic function as defined by fractional shortening, peak normalized rates of left ventricular shortening and posterior wall thinning were not altered by acute rejection. We conclude that the left ventricular function of non-rejecting hearts is similar to that of normals; however, acute rejection is associated with abnormal left ventricular diastolic dynamics without adverse effect on systolic function.