OBJECTIVES: The purpose of this study was to determine whether written advice from general practitioners increases physical activity among sedentary people more than verbal advice alone. METHODS: Sedentary patients (n = 456) received verbal advice on increasing physical activity and were then randomized to an exercise prescription (green prescription) group or a verbal advice group. RESULTS: The number of people engaging in any recreational physical activity at 6 weeks increased substantially, but significantly more so in the green prescription group. Also, more participants in the green prescription group increased their activity over the period. CONCLUSIONS: A written goal-oriented exercise prescription, in addition to verbal advice, is a useful tool for general practitioners in motivating their parents to increase physical activity.