The changes in respiratory function may be due to factors such as gender and body fat distribution in addition to the direct relationship between increased body mass index (BMI) and respiratory insufficiency. However, the predicted values in the literature are limited to eutrophic population, which does not clarify the possible influence of obesity on the Respiratory Pressures (Costa D et al., 2010). <b>Aim:</b> To investigate the influence of weight and Body Mass Index (BMI) on Respiratory Muscle Strength. <b>Patients and Methods:</b> One hundred and twenty patients with BMI 38.5 ± 12.8 kg/m² and medium age 46 ± 14 years. The Maximal respiratory pressures (Maximal Inspiratory pressure - MIP and expiratory - MEP) were determined following a standardized protocol. <b>Results:</b> Regarding MIP and MEP, the measured values were lower in theolder age groupsin both genders.Data analysis were adjusted three linear regression models (Montgomery et al, 2012.): 1- (Age); 2- (age/weight); 3- (age/BMI). For the model MEP, the age and weight explained 35.58% of the variability. ForMIP,the best modelwasage and BMIand explained13.84% of the variability. <b>Conclusion:</b> It was found with the adjusted models, there was a considerable gain for MEP and MIP to add weight or BMI, and for MEP weight was better than BMI and BMI for MIP was better than weight.