OBJECTIVE. To describe acute local reactions following intraarticular hylan injection and determine their frequency. METHODS. Retrospective review of all patients with osteoarthritis of the knee treated with hylan by 3 rheumatologists. RESULTS. Twenty-two patients had 88 injections to 28 knees. Six patients had reactions within 24 h of injection characterized by pain, warmth, and swelling, lasting up to 3 weeks. This occurrence was unpredictable. Corticosteroid injections were sometimes required. Synovial fluid cell counts were 5.0-75.0 x 10(9)/l, often with a prominent mononuclear component. Crystal studies and cultures were negative. Radiographic chondrocalcinosis was present in only 1 patient. One patient had serum antibodies to chicken serum proteins. CONCLUSION. Intraarticular hylan was associated with significant local inflammatory reactions in 27% of patients, or 11% of injections. The mechanism(s) and long term sequelae are unclear.