Prenatal toxoplasmosis infection occurs when a previously seronegative women acquires the infection during her pregnancy. In order to detect early infected women and give them the correct treatment, systematic serological screening must be applied. A total of 3049 pregnant women were retrospectively studied and controlled in our service from 05/92 to 03/94 in order to a) detect seroprevalence y b) prove the efficiency of our controls. Indirect immunofluorescence was used to measure specific antibodies and ELISA for specific IgM. The total seroprevalence was 58.9%; 41.1% of the women were seronegative and susceptible to acquire the primary infection. The prevalence in the different groups according to age, was: < 19 years old: 58.8%; 20-29 years old: 56.9%; 30-39 years old: 64.2%. Only 8.5% of the whole population showed high titles in their first control and, 55% of them were checked correctly. Two primoinfections were found. Only 5% of the seronegative women received a second control during pregnancy. It can be concluded that: a) the high-risk group is large so that it is necessary to apply a suitable and systematic serological control; b) IgM antibody detection resulted very useful in order to avoid taking the second sample; c) the follow up of the negative cases was poor, therefore it was impossible to get an adequate registration of primary contacts.