To understand the dynamics of the intraarticular acute inflammatory phase of an anterior cruciate ligament (ACL) injured knee, we analyzed the level of inflammatory cytokines (TNF-alpha, IL-1beta, IL-6, IL-8, IL-1ra, and IL-10) in joint fluid samples aspirated from 34 knees following an acute ACL injury. The samples were divided into the following five groups according to the duration from injury to aspiration: within 24 h (n=5), 2-3 days (n=14), 4-6 days (n=5), 7-9 days (n=5), 10-14 days (n=4), and 15-21 days (n=3). For comparison, 7 samples were also aspirated from 4 patients with osteoarthritis and 3 with postmenisectomy hydrops (chronic arthritis group). The highest levels of inflammatory cytokines were detected in the ACL-injury group within 24 h of the injury, and the levels decreased thereafter. While there were several patterns of decrease, nearly all of the inflammatory cytokines decreased to the level of that in the chronic arthritis group within 1 week. These dynamics are similar to those reported for inflammatory cytokines in wound fluid during wound healing, and suggest that the intraarticular healing process also progresses in ACL injured knees.