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Between-participant research has shown that high negative affectivity predicts greater activity limitations and vice versa. This study examined both between- and within-participant associations of negative and positive affectivity with activity levels using ecological momentary assessment.Participants were 25 people who had undergone joint replacement surgery 12 months previously. Participants made multiple reports of their activity and positive and negative affectivity over a single day using a computerized diary. Activity was also objectively recorded using an activity monitor. The following day, participants made a self-report of their activity over the measurement day and general positive and negative affectivity levels were recorded.Higher self-reported walking time over the whole measurement day was associated with higher general positive affectivity but not negative affectivity. However, using ecological momentary assessment, higher diary reports of negative affectivity predicted increased activity levels while positive affectivity neither predicted nor was predicted by activity.These findings demonstrate the importance of within-participant methodology in detecting subtle and immediate effects of individuals' mood on behavior that may differ from findings investigating between-participant effects over longer time periods.