A 20-year-old female presented with a painful swelling in the right knee and snapping sensation on joint motion that appeared without trauma and recurred several times. She had no history of a bleeding disease or trauma. Physical examination showed no signs of rash or temperature change or systemic or local findings of an infection. The knee was tender and knee motion was painful, with 90 degrees of flexion and full extension. The ballottement test was positive. All laboratory tests including rheumatologic and bleeding parameters were normal. Joint effusion analysis was normal except for its rusty-brown color. Magnetic resonance imaging showed synovial hypertrophy and grade 2 degeneration in the medial meniscus. During diagnostic and surgical arthroscopy, rust-colored synovial hypertrophy was noted in the suprapatellar pouch accompanied by patchy villi and nodules and cystic changes. The gross appearance of the synovium mimicked that of pigmented villonodular synovitis. Biopsy specimens were obtained from different parts of the synovium and a subtotal synovectomy was performed. The histopathologic diagnosis was reported as hemosiderotic synovitis. During a three-year follow-up, she had no pain, snapping sensation, or limitation of motion. There were no recurrent effusions. © 2010 Turkish Association of Orthopaedics and Traumatology.