Background: The measurement of respiratory muscle strength has a wide application as it allows the diagnosis of respiratory failure due to muscular failure and early detection of respiratory muscle weakness. Objective: To evaluate the contraction of respiratory muscle strength in adults with asthma by measurement of maximal respiratory pressures and check for a direct / progressive correlation between worsening of respiratory muscle strength and asthma severity. Methods: It was developed a cross-sectional study involving individuals between 18-60 years of age and clinically diagnosed with asthma. We determined spirometric values, as well as Maximum Inspiratory Pressure (MIP) and Maximum Expiratory Pressure (MEP). The patients were separated in four groups according to asthma's severety (spirometric classification) and It was checked for loss of muscle strength related to severity. Results: We evaluated 124 patients with asthma, In according to spirometrics severity scores we had 24 classified as normal, 42 as light obstructive ventilator disorder, 36 as moderate obstructive ventilator disorder and 22 as severe obstructive ventilator disorder. The correlation between the spirometric variables and MIP and MEP measurements was significant only in the group with severe obstructive ventilator disorder . There was improvement in MIP and MEP values after bronchodilator test. Conclusions: In this sample, the presence of asthma as a cause of significant alterations in respiratory muscle strength was observed only in patients classified as severe. There is a very weak linear correlation between the worsening of respiratory muscle strength and asthma severity.