Background: Previous reports addressing morbidly obese patients grouped body mass indexes (BMIs) within wide ranges, such as >40 or 40-60 kg/m(2). We aimed to investigate whether the effects of pneumoperitoneum differ in narrow ranged subgroups of BMI in morbidly obese patients. Materials and Methods: Eighteen to 65 year-old, ASA I-II 75 patients were included. The subgroups of BMI were 40 <= x < 45, 45 <= x < 50, and >= 50 kg/m(2). Plato pressure (Pp), peak inspiratory pressure, and etCO(2) were recorded beside vital parameters. Dynamic compliances (C-dyn = Vt/PIP-PEEP) were calculated. Arterial blood gases were obtained before pneumoperitoneum (t(1)), at the end of pneumoperitoneum before desufflation (t(2)), and after desufflation (t(3)). Results: Pneumoperitoneum caused similar alterations in hemodynamic parameters and respiratory mechanics in Group I (n = 24), II (n = 28) and III (n = 23). Pp at t(1) and t(2) was significantly higher in Group II and III (p < 0.017). Cdyn were significantly lower at t(1) and t(2) in Group III (p < 0.01). The number of patients who required adjustments during pneumoperitoneum was significantly higher in Group III (p < 0.01). Conclusion: The Cdyn and airway pressures in laparoscopic bariatric surgery revealed statistically significant difference between BMI >= 50 kg/m(2) and BMI of 40 <= x < 45 kg/m(2). We suggest that BMIs of 40 <= x < 45 and >= 50 kg/m(2) should not be described in one group, but grouped separately in future studies addressing respiratory mechanics.