A life-threatening T. gondii pericarditis developed in a patient with symptoms corresponding to systemic lupus erythematosus (SLE) with high concentrations of antinuclear antibodies and lymphadenopathy. The diagnosis would have been SLE-associated serositis, had not pericardial fluid been inoculated into mice, because pericarditis is frequently seen in SLE and false positive toxoplasma seroreactions may occur in ANA positive patients. High IgG T. gondii antibodies without increased IgM antibodies indicated reactivation rather than primary infection. Prolonged high-dose treatment with pyrimethamine-sulphadiazine was needed. Interestingly, the patient's SLE symptoms, including high ANA antibodies, declined to an unexpected remission after treatment for toxoplasmosis. This may not be mere coincidence, but may point to a causative role of toxoplasmosis in some cases of SLE.