This study examined stability and reliability of free-living physical activity assessed by pedometer in 69 young female college students (M age = 18.7 years, SD = 1.2, range: 18-25 years; body mass index = 23.2 kg/m[superscript 2], SD = 0.6) for two complete weeks (Week 1 and Week 2) separated by 12 weeks. During Week 1, participants took an average of 8,880 steps/day, SD = 3,219, range: 1,858-19,480; during Week 2 9,088 steps/day, SD = 3,299, range 1,736-16,837; t (68) = -0.568, p = 0.572. In both weeks, only Sunday differed significantly (repeated measures analysis of variance; Week 1: p less than 0.0001; Week 2: p less than 0.01) from all other days of the week. The computed intraclass correlation between weeks was moderate (0.72). Group mean pedometer-determined physical activity was stable across 2 weeks separated by 12 weeks in this young, healthy sample of women. Individuals within these groups held their rank order to a moderate extent over time. These findings provide important evidence of the reliability of pedometer-determined physical activity data and are of practical importance to study design. (Contains 2 tables.)