Objective: This cross-sectional, observational study was planned to compare pulmonary function, respiratory and peripheral muscle strength, and functional capacity in different functional classes of heart failure patients. Materials and Methods: Thirty-four heart failure patients, in New York Heart Association (NYHA) Class II and III (68.59±9.84 years, left ventricular ejection fraction (LVEF) =34.24±7.59%) were included. Class II and III patients’ pulmonary function was assessed using spirometry, respiratory muscle strength using a mouth pressure device, functional capacity using six minute walk test (6MWT), quadriceps femoris and biceps brachii muscle strength were evaluated using a hand-held dynamometer, and variables were compared. Student t, Chi-square, Mann-Whitney U tests, and Spearman correlation coefficients were used for statistical analysis. Results: There were no statistically significant differences in pulmonary function and respiratory muscle strength between the two groups (p>0.05). The 6MWT distance (328.35±101.56 m; vs. 480.05±104.19 m) quadriceps femoris (218.83±63.62 N; vs. 290.69±115.53 N) and biceps brachii (164.22±44.78 N; vs. 219.19±49.37 N) muscle strength of Class III patients were significantly lower than of Class II patients (p<0.05 for all). The NYHA classification system was significantly correlated with measured and predicted 6MWT distance (r=-0.59, r=-0.65, respectively), biceps brachii muscle strength (r=-0.46, r=-0.40, respectively), and % quadriceps femoris muscle strength (r=-0.43) (p<0.05). Conclusion: Functional capacity and peripheral muscle strength decrease as the illness progresses in heart failure. Pulmonary function and respiratory muscle strength are preserved. The NYHA classification system is a reliable method in the detection of changes in functional capacity and peripheral muscle strength. Changes in functional capacity and peripheral muscle strength are consistent with the NYHA classification system.