Purpose: We developed the Arm Function in Multiple Sclerosis Questionnaire (AMSQ) to measure arm and hand function in MS, based on existing scales. We aimed at developing a unidimensional scale containing enough items to be used as an itembank. In this study, we investigated reliability and differential item functioning of the Dutch version. Method: Patients were recruited from two MS Centers and a Dutch website for MS patients. We performed item factor analysis on the polychoric correlation matrix, using multiple fit-indices to investigate model fit. The graded response model, an item response theory model, was used to investigate item goodness-of-fit, reliability of the estimated trait levels (θ), differential item functioning, and total information. Differential item functioning was investigated for type of MS, gender, administration version, and test length. Results: Factor analysis results suggested one factor. All items showed p-values of the item goodness-of-fit statistic above 0.0016. The reliability was 0.95, and no items showed differential item functioning on any of the investigated variables. Conclusion: AMSQ is a unidimensional 31-item questionnaire for measuring arm function in MS. Because of a well fit in a graded response model, it is suitable for further development as a computer adaptive test. ▸ Implications for Rehabilitation • A new questionnaire for arm and hand function recommended in people with multiple sclerosis (AMSQ). • Scale characteristics make the questionnaire suitable for use in clinical practice and research. • Good reliability. • Further development as a computer adaptive test to reduce burden of (repetitive) testing in patients is feasible.