STUDY OBJECTIVE: To study the changes in PETCO2 during spontaneous and controlled ventilation in patients undergoing gynecologic laparoscopy. DESIGN: Randomized, unblinded study. SETTING: Department of Gynecology, University Hospital, Linköping, Sweden; Central Hospital, Norrköping, Sweden. PATIENTS: Forty healthy patients undergoing gynecologic laparoscopy. INTERVENTIONS: Patients were divided into 4 groups: Group 1 breathed spontaneously via an endotracheal tube, while the other three groups underwent controlled ventilation to an initial PETCO2 of 3 kPa (22 mmHg) (Group 2), 4 kPa (30 mmHg) (Group 3), or 5 kPa (37 mmHg) (Group 4). MEASUREMENTS AND MAIN RESULTS: PETCO2 levels were measured at fixed time intervals. Arterial blood gas analyses were done to compare the difference between PETCO2 and PaCO2. In Group 1, PETCO2 increased soon after insufflation and remained above 6 kPa (44 mmHg) throughout the procedure. In Groups 2, 3, and 4, PETCO2 also rose after insufflation, and an initial PETCO2 of 4 kPa (30 mmHg) was ideal, as all PETCO2 values were less than 5.5 kPa (41 mmHg). Occasional episodes of arrhythmia were seen in Group 1. However, no major adverse effects were observed in any of the groups. CONCLUSIONS: In view of the high PETCO2 levels, spontaneous breathing should be avoided during gynecologic laparoscopy, and ventilation to an initial PETCO2 of 4 kPa (30 mmHg) is recommended during controlled ventilation.