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s / International Journal of Infectious Diseases 79(S1) (2019) 1–150 63 of the study was to present human clinical cases of Coxiella burnetii infection debuting as FUO. Methods & Materials: The following methods were conducted in the study: literature search, laboratory, imaging, and statistical methods. Criteria of David T. Durack and Alan C. Street were applied for FUO definition. For the etiological diagnosis indirect immunoenzyme assay (ELISA) for antibodies detection against Coxiella burnetii were used. Coxiella burnetii phase 1 IgA/IgG and Coxiella burnetii phase 2 IgG/IgM antibodies were detected in serum by indirect immunoenzyme assay (SERION ELISA classic, Virion/Serion, Würzburg, Germany). Results: For the period of January 2008 to March 2015 nine patients with FUO caused by Coxiella burnetii were hospitalized at the Department of Infectious Diseases, Military Medical Academy, Sofia (Bulgaria). Male gender were predominant (male/female – 77.8%/22.2%), mean age was 48.78 ± 14.52 years (range: 26–67), hospital stay was 9.78 ± 2.95 days (range: 5–15), fever duration was 54.33 ± 56.23 days (range: 21–180). Laboratory investigations estimated elevation of ESR 49.11 ± 31.74 mm/h (95% CI: 13.09–111.31), CRP 37.68 ± 37.62 mg/L (95% CI: 36.07–111.42) and Fibrinogen 5.69 ± 1.59 g/L (95% CI: 2.57–8.81). Mean values of liver enzymes were in reference range. Abdominal ultrasound and X-ray demonstrated 33.3% each of them Contribution to the final diagnosis. Transthoracic echocardiography found 22.2% Contribution. Serological methods presented 100% Contribution. Conclusion: Coxiella burnetii infection was accepted as a final diagnosis based on the integrated information from the applied methods. Active search and establishment of this pathogen in case of FUO should avoid potential complications and consequences in case of untreated Coxiella burnetii infection. https://doi.org/10.1016/j.ijid.2018.11.162