Because of its subcutaneous location prepatellar bursitis is frequently complicated by an infection. Gram-positive organisms, primarily Staphylococcus aureus account for the majority of cases of septic bursitis. Local cutaneous trauma can lead to direct inoculation of the bursa with normal skin flora in patients with occupations, such as mechanics, carpenters and farmers. A 71-year-old male was admitted to our department with a history of pain and swelling of his right knee over a 20 year period. Physical examination revealed a swollen, suppurative mass with ulceration of the skin and local erythema which mimicked a soft tissue tumor at first sight. Magnetic resonance imaging of the knee revealed a 13*12*10cm well-circumscribed, septated, capsulated, fluid-filled prepatellar bursa without evidence of tendinous or muscular invasion. The mass was excised en bloc, including the bursa and the overlying skin. The defect was closed with a split thickness skin graft. The patient had 100 degrees flexion and full extension after 45 days postoperatively, and he continued to work as a farmer.