Purpose: Previous studies provided evidence for the validity of the PROMIS Pediatric measures in cross-sectional studies. This study evaluated the ability of the PROMIS Pediatric measures to detect change over time in children and adolescents with cancer, nephrotic syndrome (NS), or sickle cell disease (SCD). Methods: Participants (8–17 years) completed measures of fatigue, pain interference, anger, anxiety, depressive symptoms, mobility, upper extremity, and peer relationships at three or four time points (T1–T4). Between T1 and T2, children with cancer received chemotherapy and children with SCD experienced a pain exacerbation. Children with NS were first assessed during active disease (T2), with T3 and T4 conducted at disease remission. For the primary analysis of responsiveness, we expected better scores at T3 (recovery) compared to T2 (event) for all diseases. T1 and T4 are also expected to have better scores than T2. Linear mixed models were used and adjusted for time, gender, age, race/ethnicity, education, comorbid conditions, and disease. Results: Enrolled were 96 children with cancer, 121 children with SCD, and 127 children with NS. Fatigue, pain interference, mobility, and upper extremity scores worsened from T1 (baseline) to T2 (event) (p < 0.01), and significantly improved from T2 to T3 and T4 (p < 0.01). Similarly, anxiety and depressive symptoms significantly improved from T2 to T3 and T4 (p < 0.01). Conclusions: This study provides evidence for the responsiveness of seven PROMIS Pediatric measures to clinical disease state in three chronic illnesses. The findings support use of PROMIS Pediatric measures in clinical research.