Toxoplasma gondii can cause congenital as well as postnatal toxoplasmosis in humans and animals, and thereby elicit various kinds and grades of clinical manifestations. In the human host, especially the congenital type gives rise to serious complications and sequelae such as blindness and hydrocephalus. Serological tests such as Sabin-Feldman dye test, indirect hemagglutination test, indirect fluorescent antibody test, indirect latex agglutination test, agglutination test, enzyme-linked immunosorbent assay, and detection of circulating antigens or antibodies are popularly used for the diagnosis of toxoplasmosis. In Korea, several local surveys during the past 20-30 years have shown that the antibody positive rates of patients in general hospitals ranged from 1.9% to 7.2%. Special clinical attention should be paid to immunocompromised patients, since immunosuppression is known to activate and aggravate latent toxoplasmosis, and in such cases the patients are often led to encephalitis, coma and death.