© 2016 American Cancer Society BACKGROUND: The Patient-Reported Outcomes Measurement Information System (PROMIS) was a National Institutes of Health-funded initiative to develop measures of symptoms and function. Responsiveness is the degree to which a measure can detect underlying changes over time. The objective of the current study was to document the responsiveness of 8 PROMIS measures in a large, population-based cancer cohort. METHODS: The Measuring Your Health study recruited 2968 patients who were diagnosed with 1 of 7 cancers between 2010 and 2012 through 4 Surveillance, Epidemiology, and End Results registries. Participants completed a baseline survey (6-13 months after diagnosis) and a 6-month follow-up survey. Changes in 8 PROMIS scores were compared with global ratings of transition, changes in performance status, and clinical events. RESULTS: Measures were responsive to 6-month declines and improvements in performance status with small to large effect sizes (ES) (Cohen d = 0.34-0.71; P <.01). Mean changes and effect sizes were larger for participants who reported declines compared with those who reported improvements. Small-to-medium ES were observed in patients who reported being “a little” worse (d = 0.31-0.56), and medium-to-large ES were observed in those who reported being “a lot” worse (d = 0.53-0.72). Hospitalized participants reported significant score increases, resulting in worsening of pain (d = 0.51), fatigue (d = 0.35), and depression (d = 0.57; all P <.01). Cancer recurrence and progression were associated with smaller increases in pain, fatigue, and sleep disturbance (d = 0.22-0.27). CONCLUSIONS: The current results indicated that all 8 PROMIS measures were sensitive to patient-perceived worsening and improvement and to major clinical events. These findings will be able to inform the design and interpretation of future research studies and clinical initiatives administering PROMIS measures. Cancer 2017;123:327–335. © 2016 American Cancer Society.