In patients receiving mechanical ventilation there occasionally occurs an unintentional positive end-expiratory pressure, known as auto-PEEP, a phenomenon that can easily go undetected. Potential adverse effects of auto-PEEP include misleading hemodynamic information, cardiopulmonary morbidity, or both. Our study was designed to evaluate the incidence of auto-PEEP, its magnitude, and factors influencing it in a university hospital. Every patient receiving ventilation was evaluated for controlled ventilation and auto-PEEP in medical and surgical intensive care units during a 3-month period. Expiration was stopped at the instant that the next inspiration would have been delivered. At this moment, when the pressures in the lung and the ventilator circuit had equilibrated, the level of auto-PEEP was systematically measured. Of 57 patients undergoing controlled mechanical ventilation, 27 (47%) had 1 to 6 cm H2O of auto-PEEP. The incidence of auto-PEEP increased significantly when the minute ventilation exceeded 18.4 L/min, the respiratory rate was higher than 27 breaths/min, or the set PEEP was greater than or equal to 10 cm H2O. Therefore, auto-PEEP is the result of several ventilator-related factors. Because auto-PEEP occurred in 47% of the patients in the study, routine monitoring for auto-PEEP in patients receiving controlled ventilation is recommended. Further studies of auto-PEEP and its clinical impact are warranted.