This article presents the Mexican Institute for Social Security (IMSS) created in 1943 and describes its main features, its programmes and the role played by health education inside the programmes. It ends by explaining the present situation concerning health education and the changes which are presently envisaged. During its first twenty years, the IMSS promoted preventive medicine and trained health personnel. Since 1979 it has concerned itself mainly with some 10 million peasants and marginal groups. In the frame of a national development programme, a vaccination and detection campaign were implemented and the distribution system of potable water was extended. Mexico with a population of 73 million has the dual characteristics of a developed and a developing country (70 per cent of its population is urban, 30 per cent rural). The overcrowded cities contrast with the isolated rural areas where sanitary conditions are poor and life difficult. The main causes of mortality, in 1978, were: -in the towns: car accidents, cardiovascular diseases and suicide; -in rural areas, acute respiratory infections and intestinal infections. The 1978 Alma Ata international conference on primary health care and the meeting of Ministers of health convened in 1980 by the Pan American Health Organization endorsed the IMSS programmes which emphasize prevention, promote health education and community participation. The cost of preventive measures being cheaper than treatment, 203 million pesos were saved and allocated to the expansion of programmes. Systematic immunization has resulted in a sharp decline of diphteria, polio, rabies, typhoid, pertussis and measles. Early detection of tumours of cervix uteri has saved many lives.