Background: The Modified Pulmonary Index Score (MPIS) was developed as an indicator of the severity of acute asthma in children. The objective of this study is to evaluate the reliability and validity of the MPIS for children with acute asthma, including those five years or younger in age. Methods: We evaluated the inter-rater reliability and internal consistency of the MPIS by having at least two trained physicians and a nurse—each of whom was blinded to the others’ scores—simultaneously examine inpatients with asthma exacerbation and rate them according to the MPIS. We also evaluated the intraclass correlation coefficient (ICC), kappa, Cronbach’s α and correlations between the MPIS and other indicators associated with asthma severity. Results: A total of 25 children (median age, five years; 13 patients were five years or younger in age) were enrolled in this study. The MPIS showed excellent inter-rater reliability (all ages: ICC = 0.95, 95% CI = 0.94-0.97; five years or younger: ICC = 0.93, 95% CI = 0.89-0.96) and good internal consistency (all ages: Cronbach’s α = 0.87; five years or younger: Cronbach’s α = 0.85). The MPIS showed good correlation with a visual analogue scale assessed by the physicians. Conclusions: The MPIS was a sufficiently reliable assessment tool for children with acute asthma, including those five years or younger in age.