Digitized M mode echocardiography was used to evaluate the effect of a competitive 24-h run on the left ventricular diastolic function in 12 well-trained marathon runners who completed 146-227 km during the race. Mitral valve opening was delayed, early diastolic filling was decreased and prolonged, and posterior wall thinning was reduced, particularly among those athletes completing close to 200 km or more. Since the alterations were in part the opposite in those running 160 km or less, only the reductions in the peak rate of dimension increase (P less than 0.05) and posterior wall thinning (P less than 0.01) were significant in the group as a whole. The delay in mitral valve opening (r = 0.76), the decrease in the peak rate of dimension increase (r = -0.68), and the prolongation of the early diastolic filling period (r = 0.60) were correlated with the distance completed. The reductions in left ventricular end-diastolic dimension and fractional shortening were not in proportion to the distance run, however (r = 0.23 and 0.46, respectively). Measurements made on six athletes 2-3 days after the race showed reversal of the indices of left ventricular diastolic function. Extremely exhaustive prolonged exercise thus appears to result in a marked reversible impairment in left ventricular relaxation and filling. Since the effect of these abnormalities in cardiac filling during exercise is probably more important due to the shorter diastole, the prevention of hypohydration, which could otherwise further compromise left ventricular filling, becomes crucial.