Synovial haemangioma is a rare but important cause of knee symptoms, which, when undiagnosed, can lead to significant morbidity. Diagnosis is frequently difficult and delayed. We report on a case of synovial haemangioma, which demonstrates the difficulties inherent in diagnosis and the morbidity associated with diagnostic delay, in a young woman. Magnetic resonance imaging (MRI) is a useful tool for diagnosis, but detection on MRI can also be problematic, as shown by this case, demonstrating the need for greater awareness of this condition by both clinicians and radiologists. Arthroscopy is important in both the diagnosis and treatment of these lesions.