Hyaluronic acid has been used successfully in the treatment of osteoarthritis since 1989. There is no experience in haemophiliacs in larger study groups. In a prospective study, 20 patients (21 knees) with haemophilic arthropathy of the knee received 20 mg hyaluronic acid by intra-articular injection for 5 consecutive weeks. Assessment included clinical scores, X-ray, magnetic resonance imaging (MRI) and biomechanical motion analysis before and 3 months after the first injection. The score of the WFH advisory committee and the Aichroth score for special evaluation of the knee were used. After an average period of 26 months, the World Federation of Hemophilia (WFH) score, the Aichroth score and the visual analogue scale were evaluated again. All patients had pain caused by their arthropathy, nine of them had positive antibodies to human immunodeficiency virus, and 15 had chronic hepatitis C. The mean WFH score was 8.1 points, the Petterson score was 7.3 points and the Aichroth score was 38 points (maximum 55 points). The WFH score decreased to 7.3 points, the Aichroth score improved to 40 points and the subjective assessment measured with a visual analogue scale improved from 5.3 to 3.7 points. No differences from MRI controls were detected. After 3 months, 14 of 20 patients improved subjectively, particularly in longer walking distance, stair-climbing or initial pain. These positive aspects were limited by arthropathy in adjacent joints. After 26 months 10 patients still are benefiting for up to 31 months follow-up. The average WFH score was 7.3 points, the Aichroth score 39 points, the visual analogue scale 4.0 points. We recommend hyaluronic acid for haemophilic arthropathy of the knee when regular conservative therapy has failed and operative treatment is not feasible.