Serological parameters were compared in 15 cases of Coxiella burnetii infection comprising 5 cases each of primary Q fever, chronic granulomatous hepatitis, and endocarditis. The diagnosis was made on the basis of clinical history and serology and on the isolation of C. burnetii phase I from biopsy specimens of liver and bone marrow from two patients with granulomatous hepatitis and from the aortic valve vegetations of five patients with endocarditis. The temporal sequences of immunoglobulin levels, rheumatoid factor, and specific antibody responses to phase II and phase I antigens of C. burnetii were evaluated as predictive correlates of the three Q fever entities. Serum levels of immunoglobulin classes G, M, and A were variable in all the entities of Q fever. Increased mean levels (in milligrams per deciliter) of immunoglobulin G (IgG) and IgA were noted with chronic disease in the sera of some patients, whereas IgM levels were not significantly different from normal values. Rheumatoid factor was significantly elevated in chronic disease but not in primary Q fever. The temporal sequence of C. burnetii phase II and phase I antibodies were compared by microagglutination, complement fixation, and indirect microimmunofluorescence tests. All of these serological tests were useful in distinguishing primary from chronic disease. Thus, the ratio of anti-phase II to anti-phase I antibodies was greater than 1, greater than or equal to 1, and less than or equal to 1 for primary Q fever, granulomatous hepatitis, and Q fever endocarditis, respectively. Moreover, the high phase-specific IgA antibody titers in the indirect microimmunofluorescence test were diagnostic for endocarditis.