The pathophysiology of Q fever endocarditis is characterized by the suppression of antigen-specific cell-mediated immune responses. We investigated the production of interleukin-10 (IL-10) and transforming growth factor beta (TGF-beta), known to interfere with the development of protective cell immunity. IL-10 was markedly released by unstimulated peripheral blood mononuclear cells (PBMC) from patients with Q fever endocarditis. This release resulted from the upregulation of IL-10 gene transcription. Similarly, the release of TGF-beta1 and TGF-beta2 was significantly higher in patient PBMC than in control cells, but the expression of their respective mRNA was not enhanced in patient cells. In contrast, lipopolysaccharide-stimulated transcription and release of IL-10 and TGF-beta were similar in patients and controls. The release of IL-10 by PBMC but not that of TGF-beta was correlated with the clinical status of the patients. First, IL-10 production was correlated with specific antibody levels. Second, IL-10 release remained elevated in patients prone to relapse. Taken together, our results suggest that the release of IL-10 and TGF-beta is upregulated in Q fever endocarditis. IL-10 might be considered as a marker of disease relapses and might be instrumental in monitoring the efficiency of the treatment.