The use of the digital pedometer in monitoring the physical activity of children has become popular in recent years. Gathering representative habitual physical activity data on children using pedometers requires attention to the issues of reactivity and the number of monitoring days needed. Reactivity is defined as a change in normal activity patterns when participants’ know that their activity levels are being monitored. Vincent & Pangrazi (2002) demonstrated that reactivity is not a problem with elementary school children over eight days of weekday monitoring when sealed pedometers are used. No published research has examined the issues of reactivity and the number of monitoring days needed when unsealed pedometers are used. This study had several purposes: 1) To determine if step counts were similar for open and closed pedometers when both were worn at the same time, 2) To determine if step counts were higher for the first few days compared to later days of a seven-day monitoring period, and 3) To examine how many days of monitoring were necessary to predict seven-day activity patterns in children. Twenty-eight participants from grades 4-6 (10.3±1.0 years) wore both unsealed (12761±5193 steps/day) and sealed (13129±5089 steps/day) pedometers for seven weekdays. Results indicated no significant differences in daily step counts between unsealed and sealed pedometers (F1, 162 =.739, p=.397). Data from unsealed pedometers resulted in more lost data than for sealed pedometers and the intraclass correlations among days were higher for sealed than unsealed pedometers. Day 1 step counts were significantly higher (3421 sealed and 3408 unsealed) than for all other days of the week. Four-day mean step counts were statistically similar to seven-day counts, as well as to five and six day mean counts. The results suggest that reactivity can be a factor when using pedometers with children when the feedback from an open pedometer is present. The results also support previous research (Vincent & Pangrazi, 2002) that suggests four days of monitoring are needed to determine habitual physical activity levels in children. Investigators interested in monitoring children’s physical activity levels using pedometers must weigh the costs of time and labor with sealed pedometers against the potential for reactivity and the loss of data due to accidental pedometer resets with unsealed pedometers. Use of a delayed reset button to avoid accidental resets and a familiarization period (one day) to reduce first day reactivity are offered for consideration.Keyword(s): elementary education, measurement/evaluation, physical activity