Massively obese subjects frequently undergo pulmonary function tests nowadays. Obesity-associated decreases in key operating lung volumes (reduced inspiratory capacity and reduced vital capacity) are particularly concerning as they may shorten the "room" for tidal volume expansion with negative physiological and sensory consequences.Is massive obesity associated with reduced operating lung volumes? If so, is this effect more pronounced in men than women?and Methods: An observational, retrospective study performed in a tertiary, hospital-based laboratory. Pulmonary function tests from 248 super-obese (SO, body mass index (BMI)= 50-59.9 kg/m2) and 83 super-super obese (SSO, BMI≥60 kg/m2) men and women were analyzed. Electronic medical records were screened to ensure that subjects were free of any disease which could potentially interfere with lung volumes.The prevalence of a low total lung capacity (restriction) was 26.9%, increasing to 38.6% in SSO. Despite the absence of between-sex differences in BMI and spirometric variables derived from the forced maneuver in both SO and SSO, men presented with higher prevalence of restriction (46.7%) than women (19.4%) (p<0.05). Between-sex differences in residual volume differed according to the BMI group: whereas SO men presented with higher values than SO women, the opposite was found in the SSO group. The prevalence of restriction with low operating lung volumes was ∼ twice (SSO) and ∼ thrice (SSO) as high in men compared to women (p<0.01). Linear prediction equations as a function of sex, demographic and anthropometric attributes markedly reduced the prevalence of these abnormalities across the population.Obesity-related restriction leading to low operating lung volumes is highly prevalent in the massively obese subject, particularly in men. These alterations associated with massive obesity should be taken into consideration for an accurate interpretation of pulmonary function tests in this growing population.