Aims: The aims of this study were to investigate the relationship between functional capacity and pulmonary function, respiratory and peripheral muscle strength, and find out the best predictor of functional capacity in patients with heart failure. Design: A prospective, cross sectional study. Subjects and Methods: Thirty-four clinically stable patients with heart failure (68.59 +/- 9.85 years, left ventricular ejection fraction = 34.24% +/- 7.59%, New York Heart Association class II/III) were included. Patients' characteristics were recorded. Functional capacity was evaluated using 6-minute walk test (6MWT). Pulmonary function was measured using spirometry, respiratory muscle strength (maximum inspiratory pressure and maximum expiratory pressure) was measured using a mouth pressure device, and quadriceps femoris and biceps brachii muscle strengths were assessed using a hand-held dynamometer. Results: There were statistically significant positive correlations between 6MWT distance and pulmonary function, maximum inspiratory pressure, maximum expiratory pressure, quadriceps femoris and biceps brachii muscle strengths (P < .05). In the multiple regression analysis conducted in the 34 heart failure patients, 79% of the variance in the 6MWT distance was explained by qaudriceps femoris (R(2) = 0.70, P < .001) and the biceps brachii (R(2) = .09, P = .007) muscle strengths. When the 6MWT distance was expressed as the percentage of predicted values, 58% of the variance in the 6MWT distance was explained by qaudriceps femoris (R(2) = 0.40, P = .001) and the percent biceps brachii muscle strength (R(2) = 0.18, P = .007). Conclusions: Upper and lower extremity muscle strength is a factor significantly contributing to impaired functional capacity in patients with heart failure. These findings suggest that routine screening of upper and lower extremity muscle strengths is advisable in patients with heart failure.