Continuous assessment of mixed venous oxygen saturation (cSvO2) during exercise using a fiber optic pulmonary artery catheter can provide valuable information on the physiological determinants of the exercise capacity in patients with chronic heart failure (CHF). Since its accuracy is not well established during exercise, this study evaluated the reliability of a fiber optic pulmonary artery catheter for measuring SvO2 during exercise in CHF patients. Ten patients with stable CHF performed steady-state exercise tests at 30 and 80% of the ventilatory threshold and consequently a symptom-limited incremental exercise test. During the tests, SvO2 was monitored continuously using a fiber optic pulmonary artery catheter (CCOmbo, Edwards Lifesciences, Irvine, CA, USA) and by oximetric analysis of mixed venous blood samples obtained at rest (n = 26), steady state (n = 17) and peak exercise (n = 8). There was a significant correlation between oximetrically determined SvO2 and cSvO2 values (r = 0.97). The bias between both methods was 0.6% with limits of agreement from -8 to 9%. The limits of agreement for SvO2 values 30% (n = 35) (from -10 to 12% and from -7 to 8%, respectively). In conclusion, continuous measurement of SvO2 during exercise using a fiber optic pulmonary catheter is reliable in patients with CHF, with somewhat less accurate measurements of SvO2 below 30%.