The seroprevalence of toxoplasmosis in 832 pregnant women in Miracema, Rio de Janeiro, was determined and 75.1% (625) The parasite Toxoplasma gondii is one of the most frequent to infect human beings and is found in almost one-third of the world population (Dubey 2004). Patients are frequently asymptomatic and toxoplasmosis has no relevant repercussions on the patient. However, infected fetuses and immunodeficient individuals may present with serious or even fatal clinical outcomes (Gross 1996, Vidigal et al. 2002. Toxoplasmosis infection is transmitted (1) by the ingestion of raw or undercooked meat, which contains tissue cysts of the parasite; (2) through the accidental ingestion of oocysts eliminated in the feces of infected cats; and (3) transplacentary due to the primary gestational infection (Frenkel et al. 1970, Hutchinson et al. 1971, Aspinall et al. 2003, Bahia-Oliveira et al. 2003.The diagnosis of toxoplasmosis is based mainly on serological tests that detect anti-T.gondii IgM and IgG antibodies (Camargo 1995, Pinard et al. 2003, that correspond to acute and chronic infections, respectively. Although IgM anti-T. gondii (IgMTg) tend to disappear, it can remain at low titers for a long period of time complicating the interpretation of the serological diagnosis of the acute phase in that case they are named residual IgM and correspond to the chronic phase of the disease (Bastien 2002, Remington et al. 2004. In order to differentiate the two phases, an important step in defining In the present report, the prevalence of residual IgM in pregnant women from Miracema, Northwest of the state of Rio de Janeiro, Brazil (Table I) was investigated, as well as the correlation of these antibodies with IgG avidity, in the patients and in their family groups, in order to effectively establish the diagnosis of acute or chronic toxoplasmosis. The studied sample was chosen considering an estimated prevalence (IgGTg) of toxoplasmosis of 75% in order to identify the "n" value necessary to obtain enough seronegative individuals to ensure that 25% of the sample would be candidates for being IgMTg positive during pregnancy. Consequently, in order to determine the sample size, the population of women in the municipally (n = 13521), a prevalence of 25%, an estimated error of 3%, and a significance level of 5% were considered as parameters leading to a minimum sample of 756 pregnant women. Thus, from May 2003-December 2006, 832 pregnant women that visited the municipal public health system were selected and evaluated.Serological tests were performed in order to detect IgMTg and IgGTg antibodies using a commercial enzyme linked immunosorbent assay Diesse® (ELISA) kit in all pregnant women. This technique has 100% and 99.6% specificity and sensitivity, respectively, for detecting IgMTg and has 100% for both analytical parameters for IgGTg. The patients that were IgMTg and IgGTg posi-