Approximately 85 percent of women of childbearing age in the United States are susceptible to acute infection with the protozoan parasite Toxoplasma gondii. Transmission of T. gondii to the fetus can result in serious health problems, including mental retardation, seizures, blindness, and death. Some health problems may not become apparent until the second or third decade of life. An estimated 400 to 4,000 cases of congenital toxoplasmosis occur in the United States each year. Serologic tests are used to diagnose acute T. gondii infection in pregnant women. Because false-positive tests occur frequently, serologic diagnosis must be confirmed at a Toxoplasma reference laboratory before treatment with potentially toxic drugs is considered. In many instances, congenital toxoplasmosis can be prevented by educating pregnant women and other women of childbearing age about not ingesting raw or undercooked meat, using measures to avoid cross-contamination of other foods with raw or undercooked meat, and protecting themselves against exposure to cat litter or contaminated soil.