Objective: To evaluate the dose-response relationship between smoking load and cardiopulmonary fitness, as measured with cardiopulmonary exercise testing (CPET), in adult smokers free of respiratory diseases. Methods: After a complete clinical evaluation and spirometry, 95 adult smokers (35 men and 60 women) underwent CPET on a treadmill. Results: The physiological responses during CPET showed lower cardiorespiratory fitness levels, regardless of smoking load, with a peak V-o2(') lower than 100% of the expected value and a lower maximum heart rate. We observed a significant moderate negative correlation between smoking load and peak V-o2('). The smoking load also presented a significant negative correlation with maximum heart rate(r=-0.36; p < 0.05), lactate threshold(r=-0.45; p < 0.05), and peak ventilation(r=-0.43; p<0.05). However, a dose-response relationship between smoking load quartiles and cardiopulmonary fitness was not found comparing quartiles of smoking loads after adjustment for age, sex and cardiovascular risk. Conclusion: There appears to be no dose-response relationship between SL and cardiopulmonary fitness in adult smokers with preserved pulmonary function, after adjusting the analysis for age and cardiovascular risk. Our results suggest that smoking cessation might be useful as the primary strategy to prevent cardiopulmonary fitness decline in smokers, regardless of smoking load. Thus, even a very low dose of tobacco use must be avoided in preventive strategies focusing on becoming people more physically active and fit. (C) 2016 Sociedade Portuguese de Pneumologia. Published by Elsevier Espana, S.L.U. This is an open access article under the CC BY-NC-ND license.