Degenerative changes resembling hemophilic arthropathy may be a complication of synovial hemangioma in the knee. It is thought that arthropathy is caused by repeated bleeding episodes similar to the joint disease in hemophilia. Four children aged 4 to 9 years at surgery were treated by anterior open synovectomy for intra-articular synovial hemangiomas. At open surgery the cartilage of all patients showed changes on the surface with yellow hematin staining in all cases, without ulceration in three patients but one, who showed severe degenerative changes at age 9 years. At 15 months follow-up the patient with severe degenerative changes had severely impaired flexion and mild extension deficit. The three others were asymptomatic at 1 to 6 years follow-up. The magnetic resonance images were typical and can be considered pathognomonic; however, the referral diagnosis included the correct diagnosis in none of them. Treatment should be initiated as early as possible to reduce the risk of damage to the cartilage. Treatment by complete open synovectomy in these four patients was performed without significant bleeding problems, and no recurrence was seen.