Cervical cancer is still an important public health problem in Mexico; in spite of the development of various programs aimed at lowering mortality due to this disease, impact has not yet been achieved. In this study we present an evaluation of the Early Detection Program for Cervical Cancer in Mexico City. The authors carried out a case-control study with a populational base, in which a sample of cases obtained in eight hospitals and a sample of controls representative of the general population were included. We report a protective effect attributed to the program. The women with complete cytology showed 2.4 times less risk of cervical cancer. Nevertheless the effect of the program at a populational level is still limited because it covers only 20 per cent of the cases in the population. The results indicate the need for developing operative research programs in order to increase the demand for and the quality and efficiency of preventive services.