The authors compared two groups of 20 patients suffering from Q fever using microimmunofluorescence (micro IF) serology. One group had endocarditis and the other conventional symptoms of acute Q fever but no endocarditis. Determination of the levels of antibodies against the two phases of rickettsiae in each of the three immunoglobulin classes (IgG, IgM and IgA), allowed to determine the type of infection using a single serum sample. Patients having IgA class antiphase I antibodies at a level equal to/or higher than 1:25 as well as those whose antibody levels fulfilled the conditions for the equation (IgG anti-phase I greater than or equal to IgG anti-phase II) + (IgA anti-phase I greater than or equal to IgA anti-phase II) were suffering from endocarditis. The positive predictive value of these tests was 100% and 94.1%, respectively.