Esophageal pressure (Pes) is a surrogate for intrapleural pressure. Measuring Pes during mechanical ventilation allows for positive end-expiratory pressure adjustments by transpulmonary pressure (PL), which has been shown to improve oxygenation and outcome in acute respiratory distress syndrome patients. In morbidly obese patients, we saw progressively increasing PL measurements, although airway pressure (Paw), intra-abdominal pressure, and patient position did not change. On further examination, we determined that the gradual increases of PL were artifacts caused by a leak in the pressure probes, which resulted in underestimation of Pes and overestimation of PL as derived from the equation Paw - Pes = PL.