OBJECTIVE: To examine score agreement, precision, validity, efficiency, and responsiveness of a computerized adaptive testing (CAT) version of the Activity Measure for Post-Acute Care (AM-PAC-CAT) in a prospective, 3-month follow-up sample of inpatient rehabilitation patients recently discharged home. DESIGN: Longitudinal, prospective 1-group cohort study of patients followed approximately 2 weeks after hospital discharge and then 3 months after the initial home visit. SETTING: Follow-up visits conducted in patients' home setting. PARTICIPANTS: Ninety-four adults who were recently discharged from inpatient rehabilitation, with diagnoses of neurologic, orthopedic, and medically complex conditions. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Summary scores from AM-PAC-CAT, including 3 activity domains of movement and physical, personal care and instrumental, and applied cognition were compared with scores from a traditional fixed-length version of the AM-PAC with 66 items (AM-PAC-66). RESULTS: AM-PAC-CAT scores were in good agreement (intraclass correlation coefficient model 3,1 range, .77-.86) with scores from the AM-PAC-66. On average, the CAT programs required 43% of the time and 33% of the items compared with the AM-PAC-66. Both formats discriminated across functional severity groups. The standardized response mean (SRM) was greater for the movement and physical fixed form than the CAT; the effect size and SRM of the 2 other AM-PAC domains showed similar sensitivity between CAT and fixed formats. Using patients' own report as an anchor-based measure of change, the CAT and fixed length formats were comparable in responsiveness to patient-reported change over a 3-month interval. CONCLUSIONS: Accurate estimates for functional activity group-level changes can be obtained from CAT administrations, with a considerable reduction in administration time.