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Knowledge of the development of different classes of antibody during the course of acute Q fever is important to the clinician for interpreting a patient's serological test results. In the present study, the appearance of antibodies to Coxiella burnetii phases I and II was determined for a period of 1 year. A total of 683 sera from 191 patients with symptomatic Q fever were evaluated by the complement fixation and indirect immunofluorescence (immunoglobulins M and G [IgM, IgG]) tests. These patients had contracted acute Q fever in the fall of 1983 during an epidemic that resulted in 415 serologically confirmed cases of Q fever. As demonstrated by the complement fixation test, antibodies to C. burnetii phase II remained elevated throughout the entire study period, whereas antibodies to phase I were barely detectable. Although the immunofluorescence test was more sensitive than the complement fixation test, the specific anti-IgG response to C. burnetii to phases I and II gave the same general antibody profiles as did the complement fixation test. IgM anti-phase I and II titers appeared earlier but disappeared after 10 to 12 weeks. During this period, anti-phase II antibody levels were generally much higher than anti-phase I antibody levels.