To determine the cost-effectiveness of the previous appointment. Cost-effectiveness study. Family Medicine Unit in the Mexican Public Health System, in March and April 2008. All Family Medicine outpatient clinic offices who use the appointment system were included (n=32). The productivity, doctor and user dimensions were analysed in the two shifts (morning and evening) operated by the Unit. Productivity included the percentage of appointments (appointment requested more than 12h before the visit), spontaneous users, those seen and cancellations; the cost was adjusted for productivity. Effectiveness was measured using doctor and patient satisfaction, on a scale from 1 to 10. The cost-effectiveness analysis took into account the perspective of the patient and the doctor in the different scenarios that considered the percentage of previous appointments. The evaluation included an incremental analysis. For the doctor, the best cost-effectiveness is in the scenario of 60% previous appointments, and the worst with 50% previous appointments. The cost differences in the incremental analysis by scenarios is 15,019 euro and 76,611 euro. From the perspective of the patient (n=96), the best cost-effectiveness is in the 70% previous appointment scenario, while the worst is the one with 50%. The incremental analysis with differences by scenarios is 5,248 euro and 330,293 euro. The best cost-effectiveness of the previous appointment is achieved with appointment percentages between 60 and 70%.