AIMS--To differentiate the serological profiles of chronic (endocarditis) Q fever from the late follow up of acute cases. METHODS--Twenty patients (10 diagnosed with acute and 10 with endocarditis Q fever) were studied. Those diagnosed with acute infection were followed up from 2.5 to 88 months (mean 35.8 months). Serological variables included indirect immunofluorescence against phase I and II of Coxiella burnetii (IgM, IgG, and IgA), complement fixation and rheumatoid factor (RF). RESULTS--All patients with titres of IgA against phase I, after IgG removal, equal to or above 320 and a complement fixation value equal to or above 128 had endocarditis. No patient with acute Q fever had such a serological profile. CONCLUSIONS--The combination of IgA against phase I and complement fixation values may be sufficient to differentiate the serological profile of chronic (endocarditis) Q fever from the late follow up of acute cases.