Left atrium and/or left ventricle dilatation on echocardiography is considered to be an indication for closure of ventricular septal defects (VSD). No study has addressed the accuracy of using dilated left heart chambers when defining significant left-to-right shunting quantified by cardiac catheterization in isolated small or moderate VSDs. In this study, the relation between dilated left heart chambers, measured by echocardiography, and left-to-right ventricle shunting, quantified by cardiac catheterization, was evaluated in patients with isolated VSD. The medical records of all patients with isolated VSD who had undergone catheterization from 1996 to 2010 were examined retrospectively. Normative data for left heart chambers adjusted for body weight (BW) and body surface area (BSA) were used. The pulmonary-to-systemic flow ratio (Qp:Qs) was calculated by an oximetry technique. A total of 115 patients (mean age 7.3 +/- A 5 years) fulfilled the inclusion criteria. There was a statistically significant difference in terms of Qp:Qs between the patient groups with normal and dilated left heart chambers, when adjusted for BW and BSA (p = 0.001 and p = 0.002, respectively). But the relationships between Qp:Qs and left heart chamber sizes on echocardiography were not strong enough to be useful for making surgical decisions, as left heart chamber dilatation was not significantly associated with Qp:Qs a parts per thousand yen 2 (p = 0.349 when adjusted for BW, p = 0.107 when adjusted for BSA). Left heart chamber dilatation was significantly associated with Qp:Qs a parts per thousand yen 1.5 only when it was adjusted for BSA (for BW p = 0.022, for BSA p = 0.006). As a result, left heart chamber dilatation measured by echocardiography does not show significant left-to-right ventricle shunting, as quantified by catheterization. We still advocate that catheter angiography should be undertaken when left heart chambers are dilated in echocardiography in order to make decisions about closing small- to moderate-sized VSD.