This study aimed to examine the presence and duration of reactivity to wearing a pedometer and recording daily step counts in free-living adults. On the first visit to the laboratory, 90 participants (69% were females, age = 26.8 ± 13.0 yr, body mass index = 23.4 ± 4.0 kg·m(-2)), blinded to the study aim, were provided with a sealed pedometer (New Lifestyles NL-800) and informed that it was a "body posture monitor" (covert condition). Participants wore the pedometer throughout waking hours for 1 wk. On their return to the laboratory, stored step counts were recorded, and participants were informed that the device was a pedometer. Participants wore the pedometer unsealed (no restriction on viewing the step count display) for 2 wk, during which they recorded their daily step count in a diary (diary condition). Mean daily step counts recorded during the covert condition and during weeks 1 and 2 of the diary condition were compared using a repeated-measures ANOVA. There was a significant overall effect of study condition (P < 0.001), with post hoc analyses revealing that mean daily step counts reported during the first week of the diary condition (9898 ± 3002 steps per day) were significantly higher than those reported during the covert condition (8331 ± 3010 steps per day) and during the second week of the diary condition (8226 ± 3170 steps per day, P < 0.001). Reactivity to wearing unsealed pedometers and step count recording seems to last for 1 wk. In the absence of any intervention material, step counts return to normal levels during the second week of monitoring and therefore represent a more accurate estimate of habitual activity. These findings have important implications to both researchers and practitioners interested in the use of pedometers for physical activity surveillance and promotion.