The sterility of total parenteral nutrient (TPN) solutions stored at room temperature for up to seven days after preparation was studied. The study was conducted in two parts. In part 1, a positive control phase was conducted to validate that the filter set would capture bacteria or fungi in the TPN solutions. In part 2,300 unused TPN solutions were retrieved from patient-care areas within 24 hours of preparation. These solutions were randomized into three study groups of 24, 72, or 168 hours after preparation. Each TPN solution was filtered aseptically using an inline 0.22-micron filter. The filters were suspended in brain-heart infusion broth and incubated at 35 degrees C. Filters were visually evaluated for turbidity as an indicator of microbial contamination. Microbial growth was reported in 2 of 100 samples filtered 24 hours after preparation (group 1) and in 1 of 100 samples filtered 72 hours after preparation (group 2). No growth was detected in TPN solutions filtered 168 hours after preparation (group 3). Turbid samples were subcultured, and contaminants were identified as coagulase-negative Staphylococcus species and gram-positive bacilli in the group 1 samples and coagulase-negative Staphylococcus species in the group 2 samples. The difference in contamination rates among the three groups was not significant. TPN solutions may be stored after preparation at room temperature for an extended time (up to 168 hours) without increasing the risk to patient safety. However, at this institution, expiration dates are extended only to the time that supports practical recycling of standard TPN solutions (72 hours).