A case report of a 28-year-old mother of two children with FUO is presented. Physical examination revealed an anemic and febrile woman, who lost 10 kg of weight during the past 3 months. Furthermore, two lymphatic nodes with diameters below 1 cm were detected at the neck and inguinal region. A search for origin of fever including evaluation of foci, malignancies and laboratory investigations was primarily unsuccessful. At day 7 after admission a pericardial murmur could be heard. Echocardiography revealed a pericardial effusion, which increased up to 4 cm during the following days, leading to hemodynamic impairment and asystole. Immediate CR was successful, pericardial effusion was aspirated. Looking for etiology of fever the presence of IgM-antibodies against toxoplasma gondii by an ELISA test was possible. Therefore, toxoplasmosis was diagnosed and a treatment-regimen comprising pyrimethamin and sulfadiazin was initiated. Because of the threat to life and very high titers of C-reactive protein, antibiotic therapy (imipenem) was given additionally. An immunologic impairment was excluded by normal ratio of CD4:CD8 of lymphocytes, normal HIV-test and a nonsuspicious Jamshidi-biopsy of the bone marrow. However, in week 9 after admission lymphatic node-tumors suddenly appeared at the neck and pulmonary hilus. After diagnostic exstirpation a malignant non-Hodgkin-lymphoma (T-cell-type) was diagnosed. It is concluded that in obscure pericardial effusion toxoplasmosis should be considered and that this manifestation may be a precursor of malignant non-Hodgkin-lymphoma.