Since the vitamin thiamin is sensitive to degradation in the presence of stoichiometric concentrations of sulfite ions, the stability of thiamin in parenteral nutrition solutions containing sulfite as an antioxidant and preservative has been questioned. The usual clinical practice of mixing amino acids, dextrose, and other solutions for administration in total parenteral nutrition significantly dilutes the concentration of sulfite. We have measured the thiamin concentration in total parenteral nutrition solutions sampled from containers after administration to patients. We have also determined the time course of thiamin degradation in solutions containing two concentrations of sulfite. Thiamin content exceeded labeled amounts in total parenteral nutrition bags containing amino acid, dextrose, and multivitamin solutions. However, when multivitamins were added directly to amino acid solutions containing 0.1% (9.6 mM) sulfite, significant degradation occurred by 5 hr. Thiamin was stable for at least 22 hr when added to total parenteral nutrition solutions containing less than 0.05% (4.8 mM) sulfite.