The author has reviewed different aspects of the role of immunocompetence in the development of Q fever. Coxiella burnetii lives within the phagolysosomes of infected cells. In animals, the immunosuppression caused by either cortisone or X-irradiation reactivates Q fever. In humans, cases of Q fever are reported in immunocompromised hosts suffering from leukemia, cancer, and human immunodeficiency virus infection (AIDS). Similar data are reported with strict or facultative intracellular parasites living within the phagolysosome. Sporadic publications reported the appearance of auto-antibodies during Q fever which may change the clinical picture of the disease. The pathological findings of hepatitis diagnosed during acute Q fever and those with C. burnetii chronic endocarditis are quite different and may reflect a different immunological response to C. burnetii. These facts emphasize the importance of host factors in the clinical expression and outcome of Q fever.