The Baker's cyst is a distension of the bursa subtendinea and is caused by noninfectious knee effusion secondary to arthrosis, meniscal tears, trauma, rheumatoid arthritis, gout, or any other form of sinovitis, like rheumatoid arthritis. An infected popliteal cyst is much less common; tuberculous arthritis is exceptional, in fact only four cases are described in literature. Herein we describe an additional tbc case of a 51 years old men, who has been initially treated with cytostatic and corticosteroid agents, for a suspected rheumatoid arthritis. The review of the literature suggests the importance of a correct approach to the diagnosis, based on the analysis and culture of the synovial fluid, because delays latency of effective antibiotic therapy can result in permanent joint damage that invalidate the patients.