High-frequency ventilation (HFV) has been suggested as an alternative to standard positive-pressure ventilation with positive end-expiratory pressure (PPV/PEEP) in the treatment of infants with hyaline membrane disease. To assess the relative safety and efficacy of HFV, we compared standard PPV to HFV delivered by oscillation (HFO) and HFV delivered by a flow interrupter (HFFI) in a previously validated preterm baboon model of hyaline membrane disease. At necropsy, the livers of several animals were unexpectedly found to have severe fatty change and/or cytoplasmic vacuolization and edema. There was a strong correlation between use of HFO and the development of liver lesions. The hepatic lesion may reflect decreased cardiac output with a concomitant fall in splenic perfusion and/or alterations in normal intrathoracic pressure that increase resistance in the hepatic vasculature. Although the etiology cannot be determined precisely, these data suggest the potential for nonpulmonary complications of HFV in the immature newborn and mandate further investigation before its widespread clinical application.