PURPOSE: To investigate the effect of being underweight on the incidence of vancomycin-associated acute kidney injury (AKI) using propensity score matching analysis. METHODS: This study is a retrospective analysis of patients who received vancomycin and had their serum concentration measured at Kitasato University Hospital between January 1, 2016 and December 31, 2020. Patients were divided into underweight and non-underweight groups based on body mass index (BMI), and propensity score matching analysis was used to evaluate whether underweight affected the incidence of acute kidney injury. RESULTS: 480 patients met the selection criteria, and 111 patients from each group (BMI <
18.5 and BMI ≥ 18.5) were successfully matched using propensity score matching. After matching, there were no differences in non-physical characteristics between the two groups. The incidence of AKI was 23.4% (26 of 111) in the BMI <
18.5 group and 37.8% (42 of 111) in the BMI ≥ 18.5 group, with the BMI <
18.5 group having a significantly lower incidence. The odds ratio was 0.503 [95% CI 0.281-0.900]. CONCLUSION: This study showed that underweight patients (BMI <
18.5) had a significantly lower incidence of vancomycin-associated AKI compared to those with BMI ≥ 18.5. As there have been no previous reports on the association between underweight and vancomycin-associated AKI, this study provides novel insights.