This study assessed the effectiveness of family physicians in increasing physical activity in sedentary patients in routine practice. Family physicians (n=56) recruited 4,317 sedentary patients (2,248 for intervention group and 2,069 for control group). Intervention physicians provided advice to all patients and a physical activity prescription to the subgroup attending an additional appointment (30%). At 6 months, patients of the intervention group increased physical activity more than controls (adjusted difference of 18 min./week). The proportion of the population achieving the minimal physical activity recommendations was 3.9% higher in the intervention group than in the control group. Patients of the intervention group older than 50 years devoted more time per week to moderate or vigorous physical activity and were more successful at achieving the minimal physical activity recommendations. The beneficial effects of the intervention program were greater in the proportion of patients who also received a prescription. Therefore, family physicians can play a role in helping sedentary patients increase their physical activity level.