In physical activity research, reactivity has been defined as a change in normal activity when participants know their activity levels are being monitored (Puhl et al. 1990). Vincent & Pangrazi (2002) demonstrated that reactivity did not exist in elementary school children when pedometers were sealed. It is not known whether reactivity is a problem when pedometers are unsealed. It may be that children will react to unsealed pedometers because they can see their results (steps). The purpose of the study was to determine if reactivity existed in 4th grade children when using unsealed pedometers. Forty-five 4th grade children (21 boys, 24 girls) from two classes wore pedometers for eight days, four days with sealed and four days with unsealed pedometers. For a variety of reasons including accidental resets, forgotten pedometers, failure to wear the pedometer for the full day, and absences, only 35 participants (18 boys, 17 girls; 9.5±0.50 years) were included in the final analysis. A factorial repeated measures ANOVA indicated that there was no significant difference between the unsealed and sealed conditions (F1,29=0.73, P=0.4). These results suggest that reactivity does not exist when using unsealed pedometers. However, accidental resets during the unsealed condition resulted in loss of data and, as a result, a large number of participants (8) had to be excluded from the analyses. Only two participants were lost in the sealed condition. In conclusion, when studying activity levels in children, caution should be used with unsealed pedometers. Pedometers designed with a delayed reset button or a less accessible reset button are possible solutions. Keyword(s): measurement/evaluation, physical activity, research