The present study sought to determine the prevalence of anemia in 2,992 children, aged between 6 and 23 months, who voluntarily attended 160 Basis Health Care Units, located in 63 cities of the 5 Regional Health Coordinating areas of the State of S. Paulo, Brazil. Blood samples were collected by venous puncture and hemoglobin was measured by the cianometahemoglobin method. The WHO criteria for the diagnosis of anemia (Hb < 11.0 g/dl) were used: 59.1% of the children were shown to be anemic, with prevalence varying from 47.8% to 68.7% in the 5 RHCs. RHC 1, which comprises the Greater S. Paulo Region, presented a prevalence of anemia significantly lower than the other 4 RHCs, which cover the rest of the State. Hemoglobin levels 9.5 g/dl were found in 25.1% of the children. Anemia was more frequent in male children in male children, in those born, with a weight of less than 3,000 gr, in those who were breastfed for less than 2 months and in those that who presented some degree of energy deficient proteic malnutrition, according to Gomez's criteria. This is the first of a series of 4 articles whose purpose is the determining the prevalence of anemia in the State of S. Paulo and of testing the intervention alternatives with a view to curtailing the incidence of this pathology which today is the most prevalent nutritional disturbance in the world.