Abstract
A 12-month-old girl presented with a chronic localised mass on the knee, discomfort and gradually increasing right genu valgum. A diagnosis of pigmented villonodular synovitis was suggested by magnetic resonance imaging and confirmed at the time of surgical excision of the mass. The patient showed no recurrence after 3 years of follow up, and she required no further management. Pigmented villonodular synovitis should be considered in the differential diagnosis of chronic knee swelling in children.