BACKGROUND: This study examined the association between parenteral energy/amino acid doses and in-hospital mortality among inpatients on long-term nil per os (NPO) status, using a medical claims database in Japan. METHODS: Hospitalized patients with aspiration pneumonia, aged ≥65 years and on >7-days NPO status, were identified in a medical claims database between January 2013 and December 2018. Using multivariate logistic regression and regression analyses, we examined the association between mean parenteral energy/amino acid doses and in-hospital mortality, and secondarily the association between prognosis (in-hospital mortality, inability to receive full oral intake, re-admission, hospital stay length) among four groups classified by mean amino acid dose (No dose: 0 g/kg/day; Very low dose: >0, ≤0.3 g/kg/day; Low dose: >0.3, ≤0.6 g/kg/day; Moderate dose: >0.6 g/kg/day). RESULTS: The analysis population included 20,457 inpatients (≥80 years: 78.3%). In total, 5,920 mortalities were recorded. Increased amino acid doses were significantly associated with reduced in-hospital mortality (p <0.001). With a No dose reference level, the odds ratios (95% confidence interval) of in-hospital mortality adjusted for potential confounders, were 0.78 (0.72-0.85), 0.74 (0.67-0.82), and 0.69 (0.59-0.81) for Very low, Low, and Moderate amino acid doses, respectively. Additionally, patients prescribed amino acid dose levels >0.6 g/kg/day had shorter hospitalization periods than those prescribed none. CONCLUSIONS: Increased amino acid doses were associated with reduced in-hospital mortality. Sufficient amino acid administration is recommended for patients with aspiration pneumonia requiring NPO status.