Background : «¡Mirame!» program was launched in Chile in 1993, to promote a healthy life style at school age. Aim: To evaluate cost-effectiveness of this program. Material and methods: Fifth and sixth grade school children, from 5 selected municipalities of the Metropolitan Region were studied. The design was a quasi-experiment with an intervention (IP, 1,435 children) and reference population (RP, 1,246 children). Tobacco (T-C) and alcohol (OH-C) consumption were the indicators. A baseline survey was done in 1993 and repeated in 1996 in both groups. The criterion of Net Change was applied to assess effectiveness. For cost evaluation, an institutional perspective was considered. Direct municipal administration and «¡Mirame!» program costs were analyzed and incremental costs were calculated, using reference municipalities as controls. A univariate sensitivity analysis was done based on the beneficial discount rate and cost discount rate. Cost effectiveness coefficient was calculated. Results: The incremental cost per each boy and girl prevented from OH-C was US$ 112 (103.6-114.3) and US$ 132 (129.9-133.3) respectively. The figures for each boy and girl prevented from T-C was US$ 154 (142.7-157.4) and US$ 130 (122.5-135.2) respectively. The program caused an additional cost per child, for the city hall of US$ 11.7 in two years. Conclusions: It is possible to apply health promotion interventions in schools with a good cost effectiveness in the short run (Rev Méd Chile 2004; 132: 361-70).