OBJECTIVE Aminophylline has proven useful for treating renal failure in preterm infants. Previous reports state that aminophylline is incompatible with some neonatal total parenteral nutrition (TPN) solutions. If this is correct, administration of aminophylline doses would be complicated by the need to hold TPN and provide flush solution after each aminophylline dose. Our experience with administering aminophylline over 30 minutes concurrently with TPN was that this was not problematic. We therefore examined the in vitro compatibility of aminophylline and TPN solutions used in our neonates over a 30-minute interval to see if our policy of allowing concurrent mixing of these products was appropriate. METHODS TPN solutions (2.5 mL) were mixed with 1 mL of intravenous aminophylline 2.5 mg/mL in a glass vial. Three different TPN solutions used in our NICU were collected for the study, and five samples of each combination were prepared. Samples were watched for 60 minutes to see if precipitation occurred. RESULTS Although the aminophylline and TPN solutions were not miscible, no turbidity or precipitation was observed. CONCLUSIONS This study supports that aminophylline is physically compatible with neonatal TPN for 60 minutes.