Worsening of mitral regurgitation (MR) is sometimes observed after closure of an atrial septal defect (ASD). However, since the mechanism of this deterioration remains unclear, the aim of our study was to investigate the effect of left (LV) and right ventricular (RV) geometry on MR after transcatheter closure of ASD.We studied 27 patients with ASD who underwent transcatheter closure. Echocardiography was performed before and 6 ± 2 months after the procedure. In addition to conventional echocardiographic parameters, full volume data of the whole LV and RV heart was obtained with 3-dimensional echocardiography. MR was quantified by measuring the width of the vena contracta, and was graded as mild (< 3.0 mm), moderate (3.0 to 6.9 mm), or severe (≥ 7.0 mm).Ten patients (37%) were classified as having worsening MR and the remaining 17 (63%) as not having worsening MR. The two groups showed similar baseline characteristics, except for patients with worsening MR being more likely to be older (P = 0.009) and having a larger left-to-right shunt of pulmonary and systemic blood flow ratio (P = 0.02). It is noteworthy that the horizontal-to-vertical ratio of basal-RV at end-systole for patients with worsening MR was significantly smaller than that for patients without worsening MR (1.0 ± 0.2 versus 1.4 ± 0.2, P < 0.0001). Furthermore, multivariate analysis showed that the horizontal-to-vertical ratio of basal-RV at end-systole was the independent predictor of worsening MR during follow-up (P < 0.001).RV geometry may affect MR after closure of ASD. The pre-operative horizontal-to-vertical ratio of basal-RV is considered useful for predicting worsening of MR after closure of ASD.