Chronic exposure to tobacco smoking may damage lung and heart function. The aim of this study was to assess maximal exercise capacity and its relationship with lung function in apparently healthy smokers. We recruited 15 heavy smokers (age 47 years ± 7, BMI 25 kg/m² ± 3, pack/years 32 ± 9) without any cardiovascular or pulmonary signs and symptoms. Fifteen healthy non smoking subjects were enrolled as a control group. All subjects underwent pulmonary function tests, electrocardiograms at rest and graded cycle exercise tests. In smokers and controls, resting lung and cardiac function parameters were in the normal range, apart from diffusing lung capacity (TLCO) values which were significantly lower in smokers (p < 0.05). As compared to controls, smokers presented lower maximal exercise capacity with lower values at peak of exercise of oxygen uptake (peak VO₂), workload, oxygen uptake/watt ratio and oxygen pulse (p < 0.05) and higher dyspnoea perception (p < 0.05). Moreover, peak VO₂, maximal workload and oxygen pulse at peak exercise were related to and predicted by TLCO (p < 0. 05). Our study confirms that maximal exercise capacity is reduced in apparently healthy heavy smokers, and shows that TLCO explains some of the variance in maximal exercise.