OBJECTIVES: There is evidence that poster prompts increase stair use. The present study was concerned with the effects of poster size, poster message, and setting on stair use. DESIGN: Using a quasi-experimental design, four observational studies were undertaken in which stair and escalator use were logged during 2-week baseline periods and 2-week intervention periods. METHODS: In the first two studies, observations were undertaken in two shopping centres (total N = 30,018) with the size of poster varying. In the other two studies (total N = 37,907), one in a shopping centre and one in a train station, two poster messages were tested in both sites. RESULTS: Pedestrian traffic volume was controlled for statistically. There were significant increases in stair use with A1- and A2-, but not A3-size posters. Overall, the two different poster messages were both effective in encouraging stair use. Interactions between gender and message setting, however, reflected the fact that the 'stay healthy, save time' poster had little impact on female shoppers but was highly effective for female commuters. CONCLUSION: These results suggest that developers of health-promotion posters pay attention to poster size. They also indicate that it is insufficient to segment audiences by gender without considering the setting and motivational context.