Intraabdominal pressure is measured conventionally at end-expiration; however, the significance of forced exhalation on this measurement has not been evaluated previously. Using data from a previous prospective cohort study of the incidence of intraabdominal hypertension and abdominal compartment syndrome in medical intensive care unit patients, the authors evaluated 65 strip-chart recordings obtained from 28 patients who had measurements of intraabdominal pressure and airway pressures taken simultaneously. Forced exhalation was identified by a rise in intraabdominal pressure during exhalation. Forced exhalation was observed in 4 patients; with a mean intraabdominal pressure increase of 14.3 +/- 1.3 mm Hg at end-exhalation, compared with a decrease of -2.5 +/- 1.2 mm Hg in 24 patients without forced exhalation and absolute pressures of 28.0 +/- 6.6 versus 13.8 +/- 3.9 mm Hg (P < .001). However, there was no difference in end-inspiratory values. Forced exhalation is not uncommon in acutely ill, mechanically ventilated medical intensive care unit patients and may increase intraabdominal pressure significantly to values that exceed the diagnostic threshold for abdominal compartment syndrome.