The presence of immunoglobulin E (IgE) antibodies was determined by using the immunosorbent-agglutination assay (ISAGA) with 611 serum samples from patients with different clinical conditions to evaluate its value for the serodiagnosis of acute Toxoplasma gondii infection. By analyzing 43 consecutively drawn serum samples from 10 pregnant women who seroconverted, we could show that specific IgE antibodies seem to appear early after infection and are usually present for less than 3 to 5 months. Therefore, we assumed that IgE antibodies seem to be detectable only during the acute or reactivated stage of infection. According to our studies, the IgE ISAGA has an overall sensitivity of only 79.5%, but a specificity of 98.0%, with positive and negative predictive values of 95.5 and 89.8%, respectively. Detection of IgE antibodies in immunosuppressed patients with reactivation of latent T. gondii infection correlates with disease activity. Despite these encouraging results, one must note that IgE antibodies were not detectable in 4 of 14 patients with very recent infection proven by seroconversion. Therefore, detection of IgE antibodies seems to correlate with early acute or reactivated toxoplasmosis, whereas negative IgE results do not exclude the possibility of the acute stage of toxoplasmosis.