In this study, we investigated the influence of pneumoperitoneum and patient positioning on respiratory mechanics and blood gases during laparoscopic Nissen fundoplication. After obtaining approval from Ethical Committee, groups of 30 patients having ASA II or III physical status were included in this study. Standard general anesthesia was performed in all patients. Intraabdominal pressure was kept at the level of 12 mmHg with 2 L.min-1 CO2 flow. Ventrak respiratory mechanic device (Fl, USA) was used for measuring respiratory mechanics. Dynamic respiratory compliance (Cdyn), airway resistance (Raw), peak inspiratoty pressure (PIP) were monitored. Measurements were made in five time points: After intubation (intubation), Trendelenburg position (Trendelenburg), during laparoscopic Nissen fundoplication surgery after pneumoperitoneum in Fowler position (Fowler), immediately before the desufflation in Fowler position (Fowler-end) and after desufflation in supine position (supine). Samples of arterial blood gases were collected at the same periods. At all periods there was a significant decrease in Cdyn and arterial oxygen pressure. Although pneumoperitoneum did not exist during Trendelenburg position, the dynamic compliance (45±13 mL cmH2O-1) was found to be significantly lower than the control value. The lowest dynamic compliance value (43±13 mL cmH2O-1) was detected during the Fowler-end period. While pH decreased in Fowler, Fowler-end and supine period, arterial carbondioxide pressure increased (p<0.05). There were not any difference among other parameters. In our study, we found that pneumoperitoneum and positioning has a negative effect on dynamic compliance and oxygenation during laparoscopic Nissen fundoplication.