A case-control study evaluating the association between mental retardation and toxoplasmosis was conducted among 845 school children in Belo Horizonte, MG, Brazil. Cases (450) were mentally retarded children attendig a public school for spacial education. Controls (395) were children from the regular public school system. Clinical and anthopometric examinations and interviwes were carried out to determine risk factors for toxoplamosis and mental retardation. Diagnosis of Toxoplasma gondii infection was based upon an indirect immunofluorescent test (IFA); 55% of cases and 20% of controls were psitive. The Relative Odds of menral retardationd in children with positive sorology was 3.0 (95% CI 2.2-4.0). Maternal exposure to cats and contact with soil were associated with and increased risk of mental retardation. Retinochoroiditis was fourford more prevalent among cases than controls and was diagnosed in T. gondii IFA positive participants. Congenital toxoplasmosis, in its sibclinical form, appears to be animportant component in the etiology of mental redation, especially in high risk (lower socioeconomic) groups. The population attributable risk was estimated as 6.0 – 9.0%, suggesting the amount of mental retardation associated with this infection.