Objective: When comparing outcomes of the Functional Independence Measure (FIM™) between patient groups, item characteristics of the FIM™ should be consistent across groups. The purpose of this study was to compare item difficulty of the FIM™ in 3 patient groups with neurological disorders. Subjects: Patients with stroke (n = 295), multiple sclerosis (n = 150), and traumatic brain injury (n = 88). Methods: FIM™ scores were administered in each group. The FIM™ consists of a motor domain (13 items) and a cognitive domain (5 items). Rasch rating scale analysis was performed to investigate differences in item difficulty (differential item functioning) between groups. Results: Answering categories of the FIM™ items were reduced to 3 (from the original 7) because of disordered thresholds and low answering frequencies. Two items of the motor domain ("bladder" and "bowel") did not fit the Rasch model. For 7 out of the 11 fitting motor items, item difficulties were different between groups (i.e. showed differential item functioning). All cognitive items fitted the Rasch model, and 4 out of 5 cognitive items showed differential item functioning. Conclusion: Differential item functioning is present in several items of both the motor and cognitive domain of the FIM™. Adjustments for differential item functioning may be required when FIM™ data will be compared between groups or will be used in a pooled data analysis.