To examine whether parenteral nutrition using a mixed lipid emulsion containing fish oil improves the neurodevelopmental outcomes of extremely low birth weight infants. The study is a secondary outcome analysis of a double-blind randomized trial of 230 extremely low birth weight infants performed at a single level IV neonatal care unit (Medical University Vienna; June 2012 to June 2015). Participants received either a mixed lipid emulsion composed of soybean oil, medium chain triglycerides, olive oil, and fish oil, or a soybean oil-based lipid emulsion for parenteral nutrition. Neurodevelopment of study participants was assessed at 12 and 24months corrected age (August 2013 to October 2017) using the Bayley Scales of Infant-Toddler Development, third edition. At discharge, 206 of the 230 study participants were eligible. At 12 and 24months corrected age, 174 of 206 (85%) and 164 of 206 (80%) infants were evaluated. At 12months, there was no significant difference in cognitive (mixed lipid: median, 95 [IQR, 85-101]; soybean oil: median, 95 [IQR, 85-100]; P=.71), language (mixed lipid: median, 86 [IQR, 77-94], soybean oil: median, 89 [IQR, 79-94]; P=.48), or motor scores (mixed lipid: median, 88 [IQR, 76-94], soybean oil: median, 88 [IQR, 79-94]; P=.69). At 24months, there was again no significant difference in cognitive (mixed lipid: median, 95 [IQR, 80-105], soybean oil: median, 95 [IQR, 90-105]; P=.17), language (mixed lipid: median, 89 [IQR, 75-97], soybean oil 89 [IQR, 77-100]; P=.54), and motor scores (mixed lipid: median, 94 [IQR, 82-103], soybean oil: median, 94 [IQR, 85-103]; P=.53). Parenteral nutrition using a mixed lipid emulsion containing fish oil did not improve neurodevelopment of extremely low birth weight infants at 12 and 24months corrected age. ClinicalTrials.gov: NCT01585935.