Due to the persistence of complex social and health problems in the marginal areas of a large city, we developed an intervention by means of a research-action project with the participation of the academy, institutional and community actors, under the frame of Primary Healthcare. The purpose of the first phase was to perform a diagnosis of the health conditions, and the purpose of the second phase was the action or intervention, both guided by a focus on social determinants. The diagnosis was a prevalence study which revealed critical overcrowding and low vaccination rates, among other results. We accomplished the socialization of the diagnosis, agreeing upon actions, local alliances, and the performance of concrete improvement interventions such as: promotion and prevention fairs aimed at lowering and controlling the identified risk factors; timely access to effective drugs; outbreak control; increase in the coverage of vaccinations; and strengthening the inter-institutional actions.