Mask-applied ventilatory support was noted to cause severe head molding in infants with birthweights under 1,501 gm. To determine if this molding was detrimental to the infant, the neonatal course and autopsy findings were reviewed for 106 infants. Twenty significant intracerebellar hemorrhages were found. An association between these hemorrhages and mask-applied positive pressure ventilation was demonstrated (P = .05). This relationship was maintained when all cases of generalized bleeding dyscrasia were removed (P = .021). It is proposed that the distortional forces produced by the mask attachment, together with the buffeting effect of intermittent positive-pressure ventilation, are causally related to the production of intracerebellar hemorrhages by direct contusion, by ischemic stasis, or by altered venous drainage. The authors urge strong caution when dealing with the small premature infant in using any attachment device that causes molding of the cranial vault, particularly in the occipital area.