A cross-sectional study of the prevalence, significance, and specificity of antibodies to phospholipids (aPL) in patients with Q fever was undertaken in a university-based tertiary care medical center. The results of the lupus anticoagulant (LA) test, VDRL test, fluorescent treponemal antibody absorption test, and ELISA with different phospholipid antigens were determined for 26 patients with Q fever diagnosed by clinical and serological criteria. Plasma from four patients with Q fever and antibodies to cardiolipin (aCL) was purified by affinity chromatography in a cardiolipin column and tested against all phospholipids. For 17 patients with high levels of aCL, a modified ELISA without fetal calf serum was performed to determine if the serum cofactor was needed for aCL activity. Fisher's exact test was used for the statistical analysis. From the results of our study we conclude the following: patients with Q fever have a high incidence of aPL, with techniques with aCL or LA being the most sensitive to determine aPL (these antibodies can help diagnose Q fever presenting only as a fever); antibodies (phase II) to Coxiella burnetii and aCL are different antibodies; and the aCL activity in patients with Q fever is cofactor-independent.