The conventional surgical technique of orthotopic heart transplantation involves reconstruction of atria from donor and recipient components. The mechanical function of the atria is thus likely to be altered either by this anatomic disruption or by the autonomic denervation, which also occurs as a result of transplantation. We investigated 18 subjects with Doppler echocardiography of transmitral flow, to assess the contributions of passive and active filling of the left ventricle in six normal subjects, six heart transplant recipients, and six heart-lung transplant recipients. The ratio of passive transmitral flow to active flow was significantly higher in isolated heart transplant recipients than in normal subjects (p < 0.001), suggesting a reduced active, late diastolic component to left ventricular filling. This was not due to denervation, because the ratio in heart-lung recipients was lower than that in normal subjects (p < 0.01). These data suggest that anatomic rather than neural dysfunction is responsible for a major change in transmitral flow characteristics after isolated heart transplantation; whether the resulting change in pattern of left ventricular filling is clinically important deserves further investigation.