Wet beriberi and Wernicke's encephalopathy (WE) are caused by thiamine deficiency and are potentially lethal and serious diseases. Thiamine deficiency occurs mainly due to poor oral intake or inadequate provision of thiamine in enteral or parenteral nutrition therapy. We report cases of wet beriberi and WE that developed after surgery in a surgical intensive care unit. The first patient, who was diagnosed with wet beriberi, underwent right total mastectomy and radical subtotal gastrectomy, simultaneously. The second was diagnosed with irreversible WE, respectively, due to long-term malnutrition. In both cases, intravenous replacement of thiamine was initiated after the admission to the surgical intensive care unit. However, comatose mentality of the second patient did not improve. As a result, we conclude that, if a patient's clinical feature is suspected to be thiamine deficiency, prompt intravenous thiamine replacement is needed.