The addition of drugs to parenteral nutrition (PN) presents a high risk of physical and chemical incompatibilities. In certain patients with water restriction or limited vein access, however, it may be necessary to add medicines to PN. Although there are numerous studies into the stability of aminophylline in PN, all of them refer to periods of less than 2 days. The purpose of the present study is to determine the stability of anhydrous theophylline over 24 hours at room temperature in 24 hours parenteral nutrition and over five days (four days stored in refrigerator and 24 hours at room temperature). In addition, to assess whether its degradation is affected by the use of single-layer or multi-layer bags. The composition of PN corresponds to a standard 1,800 Kcal diet at our hospital with a concentration of 0.164 mg/ml of anhydrous theophylline. No statistically significant differences were obtained in any of the cases studied, thus demonstrating the stability of theophylline under our study conditions both after 24 hours at room temperature and after four days of storage in the refrigerator followed by 24 hours at room temperature. Nor were differences found between storing the drug in single-layer and multi-layer bags.