OBJECTIVE: To report changes in red blood cell long-chain polyunsaturated fatty acids levels in extremely low birth weight (ELBW) infants relative to duration of intravenous lipid emulsion. STUDY DESIGN: Serial blood samples were collected from 26 ELBW infants during the first 2 months of life in the neonatal intensive care unit using a prospective cohort study design. The primary outcome was the change in long-chain polyunsaturated fatty acids levels over the study period relative to a duration of intravenous lipid emulsion of either =28 days or >28 days. Secondary outcomes included parenteral and enteral nutritional exposures as well as prematurity-associated morbidities. Longitudinal regression estimated changes in fatty acid levels between the 2 exposure groups. RESULTS: Infants with >28 days intravenous lipid emulsion had 36 more days of intravenous lipid emulsion than did those with =28 days (P < .001). Docosahexaenoic acid significantly decreased over time in all infants and decreased significantly more in infants exposed to intravenous lipid emulsion for >28 days (P = .03). Arachidonic acid significantly decreased over the study period but the decrease was not related to intravenous lipid emulsion duration. Linoleic and alpha-linolenic acids had significantly larger increases over time in those with longer exposure to intravenous lipid emulsion (P < .01). CONCLUSION: Docosahexaenoic acid status of ELBW infants declined significantly in the first 2 months of life and the decline was significantly greater in those exposed to intravenous lipid emulsion >28 days compared with those exposed =28 days.