Mitral regurgitation which is more than mild in severity is usually regarded as a relative contraindication to balloon mitral commissurotomy (BMC) because it is commonly believed that it may be worsened by the procedure. The aim of this study was to investigate the effects of BMC on pre-existing mitral regurgitation. Transthoracic and biplane transoesophageal echocardiography (TTE, TEE) combined with colour flow mapping (CFM) were performed prospectively on 50 consecutive patients immediately before and within 24 h after Inoue BMC. Before BMC, mitral regurgitation (MR) was diagnosed by TEE and left ventriculography in 36 and 13 patients respectively. Angiographic MR was mild in all 13 cases. The precise origins of MR jets were carefully sought by scanning in multiple TTE and TEE views. The maximal area of colour flow MR jets detected by TEE was measured by planimetry. After BMC mean mitral valve area increased from 1.0 ± 0.3 to 1.7 ± 0.8 cm<sup>2</sup>, p < 0.0001, mean left atrial pressure and volume decreased from 23.7 ± 5.6 mm Hg to 21.6 ± 7.5 ml, p = 0.039, and from 105 ± 56 to 90 ± 46 ml, p = 0.002, respectively. MR jets as assessed by TEE CFM disappeared in 12 patients, in all of whom MR had been undetected by angiography. MR jets remained within 20% of their original sizes in 16 (44%) patients and more than doubled in only 3 patients. However, the latter had only mild angiographic MR after BMC. BMC created new MR jets, distinct from pre-existing ones, in 27 (75%) patients. Their aetiologies were commissural splitting in 24, leaflet tears in 2 and chordal rupture in 1 case. New MR jets were co-existent with old jets in 17 (47%) cases and in 10 (28%) cases old jets were replaced by new jets. The severity of angiographic MR was unchanged in 21 (58%) of the 36 patients; new jets, all originating from one or both commissures, were found in 13 (65%) patients on TEE. Angiographic MR increased by 1 grade in 11 (33%) patients; new jets were detected in 9 patients, 8 from the commissures and 1 due to chordal rupture; in only 1 of the 11 patients did the increase in MR appear to be due to a worsening of a pre-existing jet. Angiographic MR increased by 2 grades in 3 (8%) patients; new jets appeared in all 3, arising from the commissures in 2 and from a leaflet tear in 1 case. One patient with a leaflet tear sustained an increase of 3 grades in angiographic MR. The final degree of angiographic MR was nil in 13, mild in 15, moderate in 6 and severe in 2 patients. Leaflet tears were responsible for both cases of severe MR. BMC does not appear to affect pre-existing mitral regurgitation adversely in almost all patients. It may abolish trivial jets but in most cases it creates new jets alongside the old ones. Leaflet tears are responsible for severe mitral regurgitation after BMC and this is independent of pre-existing regurgitant jets.