Stair climbing is a lifestyle physical activity that uses more calories per minute than jogging. This study tested an intervention designed to promote stair climbing in a workplace. Because previous studies provide only equivocal evidence of the effects of increased stair climbing in worksites, a formal comparison of the effects of the intervention on stair ascent and descent was made. In a five-story public sector building, a 2-week baseline was followed by 6 weeks of an intervention involving a 23½- 16½-inch poster in the lobby, the same poster and six messages affixed to the stair risers between floors, and an 11¾- 8¼-inch point-of-choice prompt at the elevators. Stair and elevator choices (n = 26,806) were videotaped throughout and subsequently coded for direction of travel, traveler's sex, and traveler's load. Weight status was coded using silhouettes beside the computer monitor. A significant effect of the intervention on stair climbing was greater in those coded as overweight (+5.4% ; odds ratio = 1.33) than in individuals of normal weight (+2.5% ; odds ratio = 1.12). Although stair descent was more common than ascent, the intervention had similar effects for both directions of travel. Stair climbing at work has few barriers and seems to be a type of physical activity that is acceptable to overweight individuals. The relatively weak effect of workplace interventions compared with results for public access staircases may reflect uncontrolled effects such as the immediate availability of the elevator for the traveler.