The strength and endurance of the inspiratory muscles was assessed in 14 patients with severe mitral valve stenosis uncomplicated by other diseases. Strength was evaluated measuring the maximal static inspiratory pressure (MIP). Endurance was measured using a two minute weight incremental test, to obtain the maximal sustainable pressure (SIP), which is the highest pressure that a subject can generate to mobilize air with incremental weight during inspiration. MIP was similar to that of 8 normal subjects (110 +/- 21 and 128 +/- 27 cm H2O respectively p = NS). SIP, maximal sustainable weight and SIP as percentage of MIP were 47 +/- 10 cm H2O, 261 +/- 81 g and 43 +/- 6% respectively, all lower than in normal subjects (89 +/- 25 cm H2O, 525 +/- 167 g and 71 +/- 15% respectively) We conclude that patients with severe mitral valve stenosis have decreased inspiratory muscle endurance compared to normal subjects and this feature may be related to their decreased exercise tolerance.