Pulmonary function studies in obese individuals have alerted the pathophysiological changes due to weight gain. However, these changes are not fully explained yet. Objective: To evaluate and analyze the pulmonary function of obese adults in the city of Manaus (AM, Brazil) and to correlate the parameters of spirometry with weight and BMI. Method: A descriptive cross-sectional study, in which pulmonary function of 22 adults with grade III obesity was evaluated pre and post bronchodilator administration, performed at the Adriano Jorge Hospital Foundation—FHAJ. Results: Of the 22 patients evaluated, 6 were male and 16 were female; mean age was 37.1 ± 7.4 years, with a weight of 134.2 ± 28.6 kg and a BMI of 51.1 ± 9.4 kg/m², corresponding to grade III obesity. Spirometry showed a reduction in values obtained from Forced Vital Capacity (FVC), Forced Expiratory Volume in the first second (FEV1) and Peak Expiratory Flow (PEF) in relation to those predicted, with 23% of restrictive disorders. There were negative correlations between BMI and PEF, weight and FVC, weight and PEF. The P value for correlation of weight and FEV1 is not statistically significant. Conclusion: Restrictive and obstructive disorders are prevalent in the population with grade III obesity. Correlations between pulmonary function and BMI/weight were negative for this specific population.