Intravenous fat (Intralipid) is used extensively as a major component of parenteral nutrition for patients in the neonatal intensive care unit. Abnormalities of polymorphonuclear leukocyte (PMN) and platelet number or function related to Intralipid infusion have been reported although conflicting results exist. In order to examine potential adverse hematologic effects of Intralipid, 10 ill neonates were studied before and after a 16-hr infusion of 1 g/kilo of Intralipid. PMN count, chemokinesis, chemotaxis, and aggregation were unchanged pre- and post intralipid infusion. Platelet count, bleeding time, and platelet aggregation were also unchanged. Similar results were obtained in vitro when neonatal and adult PMNs and platelets were incubated in Intralipid and their function analyzed. These findings suggest that short-term, low-dose Intralipid has no measurable impact on neonatal PMN or platelet activity and support its use in neonates even in the presence of infection or thrombocytopenia.