The purpose of this study was to objectively assess exercise tolerance before and after cardiac valve surgery by using an objectively determined ventilatory anaerobic threshold (AT). Nine patients (mean age: 38.2±8.1 years) with predominantly mitral regurgitant lesions were studied by a symptomatic maximal treadmill exercise test which included a determination of AT. The mean lengths of time from preoperative exercise testing to cardiac surgery, and from surgery to postoperative exercise testing were 5.9±4.0 and 12.1±8.3 months, respectively. The determination of AT on data plots was performed after blinding to patient identification and pre- vs postoperative status. After surgery, the clinical symptoms and NYHA class improved significantly with a decrease in the cardio-thoracic ratio and echocardiographic diastolic dimensions. The mean peak VO₂ (ml/kg/min) increased significantly from 20.2±7.1 to 29.7±7.9 (p<0.01). Together with these changes, AT (ml/kg/min in VO₂) increased from a mean of 14.8±4.8 to 22.8±5.5 (p<0.01). In conclusion, symptomatic improvement and an increase in peak oxygen uptake after cardiac valve surgery were accompanied by a significant increase in the objectively determined AT. AT determined in a blind manner provides an objective means of evaluating exercise tolerance when a double-blind intervention cannot be performed.