This is a case report of septic arthritis of the knee due to Prevotella loescheii, in a patient with advanced arthrosis. Two weeks beforehand he had undergone a dental root extraction without antibiotic prophylaxis. His knee had become inflamed 48 hours after extraction and he was started on ibuprofen and steroid treatment (prescribed by his primary health care doctor). With a provisional diagnosis of septic arthritis, synovial fluid was taken for study. Antimicrobial therapy was commenced with amoxicillin/clavulanic acid and the patient progressed satisfactorily. Prevotella loescheii was identified by anaerobic culture. A site of origin for the infection was never found. Joint infection is generally secondary to haematogenous dissemination of bacteria from habitual sites such as odontogenic locations. We suggest that patients with inflammatory arthropathies should be considered as candidates for antibiotic prophylaxis in oral surgery and invasive dental procedures. We suggest, in these cases, the use of antibiotic with spectrum against aerobic and anaerobic bacteria.