Propionic acidemia is often manifested during the neonatal period with vomiting, failure to thrive, lethargy, and hyperammonemic coma when catabolism is prolonged. Mild lactic acidosis frequently accompanies metabolic decompensation. We present two patients with propionic acidemia whose initial manifestation was complicated by severe lactic acidosis caused by thiamine deficiency, which resulted from an inadequate supply of, and an increased need for, thiamine during metabolic stress. To prevent acute thiamine deficiency, we propose early vitamin supplementation during treatment of any severe metabolic decompensation accompanied by insufficient food intake.