Eight preterm infants who died after 'Intralipid' infusion had fat accumulation in the lungs. The rate of infusion in six of the infants was below the recommended maximum for preterm infants and in no case was the plasma lipaemic on regular visual inspection. Histological examination revealed varying degrees of lung involvement. The commonest finding was distension of empty pulmonary capillaries, but specific staining techniques for fat showed that the capillaries were engorged with large lipid globules. Removal of accumulated fat by histiocytes was seen in infants dying some time after cessation of intralipid infusion. Analysis of homogenised lung showed that those who died after intralipid infusion had a significantly greater (p < 0.001) concentration of lioleic acid, a marker for intralipid, than infants who died without receiving parenteral fat solution. Fat accumulation after intralipid infusion may be common but unrecognised and may seriously exacerbate ventilation/perfusion inequalities.