Using a new experimental paradigm to evaluate physical activity in the natural environment, the authors made of 45,694 observations of persons using stairs or an adjacent escalator at a shopping mall, train station, and bus terminal. In study 1, stair use more than doubled for both obese and nonobese persons during two-week periods when a colorful sign encouraging use of the stairs was positioned at the stairs/escalator choice point. In study 2, stair use remained elevated for 15 consecutive days while the sign was present, decreased during a 1-month follow-up period, and returned to baseline by 3 months. These results not only demonstrate the usefulness of this paradigm, but also suggest the strength of simple, inexpensive public health interventions to increase physical activity.