The clinical and immunological 8-year follow-up of a 45-year old female with hypereosinophilic syndrome and cardiac involvement of Löffler's fibroplastic endocarditis which was complicated by infective endocarditis during immunosuppressive treatment is presented. All 3 stages of the disease were documented by clinical and histologic data: stage 1 by biopsy proven eosinophilic myocarditis and arteritis, stage 2 with Berlin blue positive parietal thrombosis, and the fibrotic features of stage 3 both by fibrosis in the latest biopsies and by echocardiography. It was remarkable, however, that this classic clinical case of Löffler's endocarditis lacked a few serological markers postulated to be found regularly, e.g., a positive staining for the cationic protein and major ribonucleases. Instead, the patient demonstrated all the immunological features of autoreactive myocarditis with cytolytic, complement fixing antimyolemmal antibodies. After an initial loading dose of 120 mg prednisolone per day for 6 weeks (11/1986), the steroid dosage was reduced to 40 mg (12/1986) and diminished to 15 mg/day in 1988 to 1989. Finally, the patient was on 4 mg prednisolone per day for almost 4 years. In 1987, azathioprine was added in the dose of 150 mg/day for 6 weeks. In 3/1994 the patient developed infective endocarditis with streptococcus sanguis and presented with dyspnoea. By echocardiography a large floating structure was diagnosed on the anterior mitral leaflet and the left atrium was enlarged by severe mitral regurgitation. Infective endocarditis was successfully treated with antibiotics, but mitral regurgitation made valve replacement obligatory.