The activity questionnaire most frequently used in heart failure, the New York Heart Association Classification, does not correlate well with peak oxygen uptake. The correlation of this variable with other activity questionnaires was analyzed in 83 patients (69 with heart failure and 14 control patients), 61.5 11 years old, who were interviewed and classified according to the Canadian Cardiovascular Society Classification. The Dyspnea-Fatigue Index and functional capacity according to the Specific Activity Questionnaire were determined for each patient. Subsequently, the treadmill cardiopulmonary exercise test (Naughton) was performed and the following correlations with peak oxygen uptake were found: Canadian Cardiovascular Society, r = 0.39; Dyspnea-Fatigue Index, r = 0.44; Specific Activity Questionnaire, r = 0.38. p < 0.001 for all three. The Dyspnea-Fatigue Index yielded the best correlation, although it was only slightly better than the Canadian Cardiovascular Society Classification, which is easier to obtain.