Body: The factors that predict exercise capacity in CHF are not well understood. It is clear the systolic factors such as ejection fraction (EF) are poor predictors of exercise capacity (VO2peak) in CHF. Recently some studies have suggested that other cardiovascular factors such as diastolic function and left ventricular stiffness may be better predictors of VO2peak. We have recently developed a method of measuring lung-to-lung circulation time (circ time) using the reappearance of end tidal acetylene (PetC2H2) in the expired air following a single inhalation. This measure of circ time is not only dependent on cardiac function but also on blood flow distribution and thus may provide additional information on the cardiovascular impairment to exercise in CHF. We hypothesized that resting circ time may be a predictor of VO2peak in CHF, with longer resting circ times indicating poorer exercise capacity. Twenty-one CHF patients (age 58Რyr, NYHA = 1.9ᰮ2; EF = 27.5Ხ0 %, meanᓅM) participated in the study. Exercise capacity was determined using an incremental treadmill test, while resting circ time was measured using acetylene inhalation. The mean peak exercise capacity (VO2peak) for the CHF patients was 18.7ᱮ1 ml.kg-1.min-1 and the resting circ time was 53.7ᱮ8 s. Peak exercise capacity was strongly correlated with resting circ time (r=-0.74, P<0.01) and poorly correlated with EF (r=0.11, p=ns). These results suggest that the measurement of resting lung-to-lung circulation time may be a better predictor of exercise capacity in CHF than more traditional cardiovascular measures of systolic function.