Musculoskeletal ultrasound (US) is gaining an increasing role in the assessment of a variegate set of joint and periarticular soft tissues abnormalities in rheumatology. In addition, US represents a useful technique in guiding local procedures such as aspiration of fluid collections and injections within the joints and periarticular musculoskeletal structures. We recently performed US in a patient with unilateral painful, swollen and erythematosus knee. Pre-patellar bursitis was demonstrated, without any signs of intra-articular abnormalities. US-guided aspiration of local effusion was performed; synovial fluid analysis demonstrated septic bursitis. Appropriate treatment determined the complete remission.