The Q-1 interval has been studied and plotted against cycle length (preceding R-R interval) in 41 patients with atrial fibrillation. Employed in such a manner, the Q-1 measurement is a useful adjunct in the study ofatrial fibrillation when mitral stenosis is present or suspected. In patients with mitral stenosis, the usual finding was an inverse relationship between the Q-1 interval and the preceding cycle length, i.e., as the cycle length increased, the Q-1 interval decreased. Severe degrees of stenosis appeared to cause characteristic variation of the Q-1 interval even at relatively long cycle lengths, reflecting the longer persisting pressure gradient across the mitral valve. In the absence of mitral stenosis, there generally was no such Q-1 variation, particularly with cycle lengths of longer than 0.6 second.