In front of a patient with arthritis, clinical good-sense tells that the most probable diagnosis are the most prevalent ones. Nevertheless, we have to exclude a multiplicity of other aetiologies, less frequent, but with highest implications in the therapeutic conduct. Infections by Brucella and by Borrelia are rare causes of chronic arthritis, yet are diagnosis to consider, even when the clinical manifestations aren't the most typical, as there still exist endemic areas in Portugal. Here we report two clinical cases about patients with arthritis for more than one year, subject to ineffective exams ant treatments. Only the clinical history could put on evidence clinical-epidemiological data, suggestive of Brucellosis and Lyme Disease, namely the professional contact with infected animals, and the history of probable erythema migrans, that pointed toward the correct diagnosis. So, with directed therapeutic, there was complete resolution of the inflammatory symptoms.