The mechanism of mitral valve area increase by double balloon mitral valvotomy in vitro has not been defined, nor have the mitral valve area results achieved by single versus double balloon mitral valvotomy technique been compared. After a selection of 29 intact mitral valves excised at cardiac surgery from patients with a mitral valve area less than or equal to 1.5 cm2 was made, double balloon mitral valvotomy was attempted in 14 valves using two 20-mm diameter balloon catheters (group 1) and single balloon mitral valvotomy using a 20-mm balloon was undertaken in 15 valves (group 2). In group 1 the mitral valve area increased from 0.9 +/- 0.03 to 1.9 +/- 0.05 cm2 (mean +/- standard error of the mean) (p less than 0.001), with a mean anterior commissural split of 5.3 +/- 0.2 mm and a posterior split of 4.1 +/- 0.2 mm. Following single balloon valvotomy (group 2), the mean mitral valve area increased from 0.8 +/- 0.03 to 1.2 +/- 0.03 cm2 (p less than 0.001), with the mean anterior commissural split being 2.6 +/- 0.2 mm and the posterior 2.1 +/- 0.2 mm. Ten mitral valves from group 2 underwent a second dilatation using the double balloon technique and the mitral valve area increased further from 1.2 +/- 0.06 to 1.9 +/- 0.06 cm2 (p less than 0.001). Overall, commissural splitting occurred preferentially in calcified commissures (81%), as opposed to only 56% of noncalcified commissures. Commissural splitting is the manner in which mitral valve area increases after double balloon mitral valvotomy.(ABSTRACT TRUNCATED AT 250 WORDS)