In persons with disabilities, the accuracy of self-report data collection methods in travel behaviour studies may be influenced by disease-related dysfunctions. The present study determines in detail whether disease-related physical, cognitive or psychosocial impairments contribute to the reporting rate of subjective self-report travel diaries and objective GPS tracking devices, besides sociodemographic and trip-related characteristics. The reporting rate of both data collection methods was analysed in 108 persons with Multiple Sclerosis with various disability severities during a seven days data collection period. The results demonstrated that there was only limited influence of diseaserelated dysfunctions on the reporting rate of both data collection methods, as well as only limited significant differences between subgroups with various disability severity. Overall, the data quality of the diary was higher than the quality of the GPS data: 66% of the trips were reported in both data collection methods, while overall more than one fifth of all trips were forgotten to be registered by GPS, and 11% were forgotten to be reported in the diary. Self-report travel diaries seemed to be more suitable for persons with a higher disability severity, as these persons more often forgot to take their GPS device with them when making a trip due to a number of organizational issues because of their mobility limitations. Neven, De Schutter, Wets, Feys and Janssens 1 INTRODUCTION Physical activity (PA) levels are often measured in order to identify current levels and changes in PA of specific population groups, and to evaluate the effectiveness of interventions designed to increase activity levels (1); which is an important part of health promotion research (2). The measurement method may have a significant impact on the observed PA levels, as self-report PA measures generally differ with direct, objective measures of PA (1). While self-report methods like questionnaires, interviews or diaries are often preferred as they are cheap to administer and offer additional information on the context of activities, objective measurement devices like pedometers or accelerometers provide more accurate measurement of PA and are not prone to certain biases in selfreport methods such as recall bias (3). Especially in research studies of persons with disabilities (PWD), the accuracy of self-report surveys for measuring PA can be influenced by cognitive dysfunctions like memory or emotional problems (4). Commonly used activity questionnaires and diaries rely on recall and honest reporting, and require individuals to have no or limited cognitive deficits to reduce potential bias in reporting results (5). According to research conducted by (6), (7) and (8), self-report subjective PA questionnaires show only limited reliability and validity regarding the frequency of different activity types (e.g. respondents tend to indicate their highest recent frequency of participation) and the duration and amount of PA. On the other hand, data of objective mon...