This article addresses food insecurity among urban Brazilian families with children under seven years. A cross-sectional study in areas covered by primary health care centers identified 5,419 families in the Northeast and 5,081 in the South of the country. Food insecurity was assessed by the Brazilian Food Insecurity Scale. Prevalence of moderate or severe food insecurity was 22.9% in the Northeast and 7.5% in the South. According to the adjusted analysis, increased likelihood of moderate or severe food insecurity was associated with families headed by women, black or brown maternal skin color, low maternal education, low family income, and enrollment in the Bolsa Família program (conditional income transfer). Moderate or severe food insecurity would be reduced by 59.5% in the Northeast and 45.4% in the South with a per capita income of at least BRL 175.00 per month. Increased family income for the poorest families and better targeting of Bolsa Família are essential for reducing food insecurity in the country.