The association between advanced age and the thiamine concentration has not been conclusively determined. A recent report from Japan showed that more than half of nursing home elderly residents at an institution had a low whole-blood thiamine concentration (<20 ng/mL). Therefore, a high incidence of low thiamine concentrations among hospitalized elderly has been anticipated in the Japanese population but never investigated. We evaluated the whole thiamine concentration in newly hospitalized elderly patients (≥65 y old) with infectious diseases. Evaluations were performed on admission and at days 6-8 of hospitalization with liquid chromatography tandem mass spectrometry (LC/MS/MS). As a result, we enrolled a total of 471 patients from September 2015 to December 2016. The median thiamine concentration was 46 ng/mL (IQR, 37-58 ng/mL). Only 7 patients (1%) had thiamine concentrations below 20 ng/mL (66 nmol/L) on admission. Five of these patients were bedridden and unable to eat food by themselves, and the other two patients used loop diuretics for chronic heart failure. The thiamine concentration declined in most patients (84%) at days 6-8 of admission, regardless of their dietary intake during hospitalization. In conclusion, a low thiamine concentration was not prevalent among newly hospitalized elderly patients with infectious diseases. However, the thiamine concentration significantly decreased during the 6-8 d of hospitalization.