AIM OF THE STUDY: The purpose of this study was to analyse the clinical and serological follow-up in 21 patients with Q fever endocarditis in Switzerland from 1981 to 1993. PATIENTS AND METHODS: Criteria for Q fever endocarditis were the following: Coxiella burnetii phase I IgG > 1 : 2560 and IgA > 1 : 20 by indirect immunofluorescence. Methods to confirm the diagnosis include immunohistochemical demonstration of C. burnetii by microscopy in valvular material (1 case) and inoculation of this material in experimental animals (10 cases). Information on clinical course of the disease, laboratory abnormalities and treatment were obtained by chart review and a questionnaire sent to physicians who requested the serological tests for Q fever. RESULTS: The average age of the patients was 47 years (15 men and 6 women). 64% of patients had a history of environmental exposure to C. burnetii. The median time of symptomatology before diagnosis was 5 months (1-108). 19/21 patients had valvular lesions, and 2/21 vascular Dacron prosthesis. Most patients presented with fever (18/21), congestive cardiac failure (14/21), weight loss (12/21), anemia (6/19), or thrombocytopenia (6/19). All the patients required antibiotic treatment. Cardiac surgery was performed in 15/21 patients. For 10 patients the geometric mean serological follow-up included at least titers at time of diagnosis (IgG anti-phase I antibodies 1 : 27024, IgA anti-phase I antibodies 1 : 685), at the end of therapy (IgG anti-phase I antibodies 1 : 2941, IgA anti-phase I antibodies 1 : 153) and 6 months after the end of therapy (IgG anti-phase I antibodies 1 : 368, IgA anti-phase I antibodies 1 : 40). The fall in anti-phase I titers was significant. During the clinical and serological outcome (median of 60 months and 69 months respectively) there was no recurrence of endocarditis and antibody titers to C. burnetii phase I remained low. Two patients died during the observation period, one from lung cancer, while the cause of death in the other was unknown. CONCLUSIONS: Serology is the key to Q fever diagnosis. The duration of treatment, and the values to be used to establish cure of endocarditis, are not clearly defined. During the clinical and serological outcome (median of 60 months and 69 months respectively) there was no recurrence of endocarditis and antibody titers to C. burnetii phase I remained low.