Acute respiratory syncytial virus (RSV)-induced bronchiolitis is often associated with continuing respiratory symptoms following hospitalization. To date, there is no validated objective measure to evaluate symptoms of RSV-induced bronchiolitis. We report on the reliability, validity, and responsiveness of the bronchiolitis caregiver diary (BCD) of symptoms and healthcare utilization associated with postacute RSV. The BCD measures four symptoms (daytime cough, wheeze, trouble breathing, and nighttime cough), healthcare utilization, and rescue medication for worsening of lung symptoms. Data from the 4-week treatment period of the reported prospective, placebo-controlled trial of montelukast for treatment of postacute RSV were used to assess reliability (internal consistency and test-retest), construct validity (cross-sectional and longitudinal correlations), discriminant validity (known-groups analyses), and responsiveness. The primary outcome of this study was the percentage of symptom-free days (SFD). The secondary outcome was a composite symptom score (CSS; average of daytime cough, wheezing, and trouble breathing). Cronbach's alpha of 0.85 indicated that the four symptoms were internally consistent, supporting a unidimensional scale structure. Test-retest reliabilities for the percentage of SFD and CSS were above the recommended cut point of 0.70. Cross-sectional and longitudinal correlations were sizeable and statistically significant, demonstrating construct validity. Hypothesized known-group differences were statistically significant in the appropriate direction. Responsiveness analyses indicated moderate effect sizes for percentage of SFD. In conclusion, the BCD provides a valid, reliable, and responsive tool for the assessment of symptoms of postacute RSV-induced bronchiolitis, capable of measuring moderate effect sizes, and demonstrating responsiveness to therapy.