Protozoal infections such as toxoplasmosis are known complications in heart transplant recipients. Diagnosis of the disease is often difficult. This article describes the course of a patient who had a febrile illness with leukocytosis and neurologic disorders after heart transplantation; all microbiologic and serologic tests of the peripheral blood and the cerebrospinal fluid failed to identify the responsible pathogen. Infection with Toxoplasma gondii was finally diagnosed by endomyocardial biopsy. We conclude that in heart transplant recipients with infections of unclear origin and neurologic disorders, endomyocardial biopsy may be helpful in the diagnosis, especially in cases of toxoplasmosis.