A seven-year-old boy fell against a blackthorn bush and found that thorns had penetrated the skin over the left knee. Aseptic synovitis developed with repeated febrile episodes. With the suspicion of septic arthritis, the patient was treated with antibiotics on repeated occasions, two negative joint aspirations and a knee joint exploration were performed. Three months after the injury the correct diagnosis was made at a second arthrotomy when an intra-articular thorn was removed and synovectomy carried out, after which healing was uneventful.