The Functional Independence Measure (FIM) is an 18-item, 7-level scale developed to uniformly assess severity of patient disability and medical rehabilitation functional outcome. FIM interrater reliability in the clinical setting is reported here. Clinicians from 89 US inpatient comprehensive medical rehabilitation facilities newly subscribing to the uniform Data System for Medical Rehabilitation from January 1988-June 1990 evaluated 1018 patients with the FIM. FIM total, domain and subscale score intraclass correlation coefficients (ICC) were calculated using ANOVA; FIM item score agreement was assessed with unweighted Kappa coefficient. Total FIM ICC was 0.96; motor domain 0.96 and cognitive domain 0.91; subscale score range: 0.89 (social cognition) to 0.94 (self-care). FIM item Kappa range: 0.53 (memory) to 0.66 (stair climbing). A subset of 24 facilities meeting UDSMR data aggregation reliability criteria had Intraclass and Kappa coefficients exceeding those for all facilities. It is concluded that the 7-level FIM is reliable when used by trained/tested inpatient medical rehabilitation clinicians.