The physical stability of a neonatal parenteral nutrition formulation has been examined using differential interference contrast (DIC) microscopy. In vitro studies indicated that particle size increases occur immediately after mixing the Intralipid emulsion with the amino acid/glucose solution, while simulation of clinical administration indicated that larger droplets were observed at the end of the catheter approximately 1 h after administration commenced. Microscopic observation of adjacent droplets of the two fluids showed reversible aggregation occurring almost immediately. It was concluded that the current method of administering this neonatal emulsion does not prevent droplet coalescence.