Currently percutaneous transvenous mitral commissurotomy (PTMC) is performed by most operators using stepwise inflation of the Inoue balloon beginning at a small diameter and increasing size by a sequence of inflations to nominal diameter. The effect of balloon size on procedure results using the Inoue balloon has not been evaluated. In 260 patients final balloon inflation was to nominal size in 42% of pts, larger than nominal size (overinflated) in 10%, and less than nominal size (underinflated) in 48% of pts. The average number of inflations was 3 (range 1-9). Pressure gradient and Doppler mitral regurgitation (MR) were assessed after each inflation. When MR increased, further inflations were not done. Patients in whom the balloon was overinflated underwent more balloon inflations than those in whom it was inflated to nominal size (5.0 +/- 2.1 vs. 2.9 +/- 1.4, p < 0.01). Those with balloon underinflation had fewer balloon inflations (2.4 +/- 1.2 vs. 2.9 +/- 1.4, p < 0.01). The post PTMC valve area in patients with overinflation was slightly less than those with nominal inflations (1.6 +/- 0.4 vs. 1.8 +/- 0.7), and with underinflation the valve area was no different compared to nominal inflation. Using a stepwise procedure, if mitral regurgitation was noted to increase after a balloon inflation, the procedure would be stopped even if less than an ideal result was achieved. Fewer inflations were done in patients in whom the balloon was inflated to less than nominal size because of the appearance of mitral regurgitation.(ABSTRACT TRUNCATED AT 250 WORDS)