Background: There may be difficulties in the use of self report measurements in patients with cognitive impairment or serious mood disturbances which interfere with reliable self assessment, as may be the case in multiple sclerosis (MS). In such cases proxies may provide valuable information. However, before using any questionnaires in a proxy sample, the questionnaire should be evaluated for proxy use. Objective: To evaluate the psychometric properties of the 29 item Multiple Sclerosis Impact Scale (MSIS-29) when used by proxies of MS patients. Methods: A sample of 62 partners of MS patients completed the MSIS-29. The data were evaluated for the psychometric criteria of the MSIS-29, including data quality, scaling assumptions, acceptability, reliability, validity, and responsiveness. Results: Psychometric evaluation was satisfactory; data quality was high, and scaling assumptions and acceptability were good. Reliability was high (α>0.80). Findings were consistent with results of a psychometric evaluation in a patient sample. Conclusions: The MSIS-29 can be used reliably in proxies of patients with MS. As a next step the relation between data obtained from patients and proxies needs to be studied, focusing on factors that may affect agreement and discrepancies.