Background: The role of diastolic ventricular function before and after Fontan operation has not been well understood. Considering the unique hemodynamics of the Fontan circulation, it may affect the acute postoperative course. We studied diastolic function immediately before and after Fontan operation.Methods: Seven patients who underwent Fontan operation (4 with LV morphology and 3 with RV morphology, mean age 2.7 +/- 0.4 years) were studied. Six patients with subpulmonary VSD with least shunt served as a control group. Ventricular pressure was recorded with a catheter transducer and simultaneous direct echocardiography was done 20 minutes before and after bypass. Relaxation rate (Tau) using the monoexponential model with a non-zero asymptote was derived from the pressure tracing. Myocardial stiffness constant (Km) in LV morphology heart was measured from diastolic dimension-stress relation. Rate-corrected mean velocity of circumferential shortening (mVcfc) was measured as a systolic performance.Results: Tau was not different between groups before bypass which significantly elongated after bypass in Fontan group, whereas it did not change in the control group. Km was not different between the groups before bypass which significantly increased after bypass in Fontan group and did not change in the control group. mVcfc was significantly lower in Fontan group before bypass and did not change in both group after bypass. Thus, ventricular relaxation was impaired and myocardial stiffness increased after bypass in Fontan group compared with the control group.Conclusions: Impaired early relaxation as well as myocardial stiffness may have affected the hemodynamics in the early postoperative phase after Fontan procedure.