Toxoplasma gondii has long been recognized as a potential cause of severe disease in the congenitally infected infant and the immunocompromised host. This report describes three children with toxoplasmosis after heart transplantation and reviews the cases of 18 adult recipients of cardiac transplants (reported in the English-language literature) who developed toxoplasmosis postoperatively. Onset of disease was within the first 6 1/2 months following transplantation. Severity ranged from asymptomatic seroconversion to myocardial infiltration or disseminated neurological disease and death. Only one patient was known to be seropositive for antibody to T. gondii prior to transplantation. Transmission was most likely via the donor organ. Seronegative patients who receive organs from seropositive donors are at high risk for serious disease; prophylactic strategies need to be developed.