The usefulness of the results of serial cultures of synovial fluid in assessing the response to medical therapy of acute bacterial joint infections was examined. A five-year retrospective survey disclosed 32 synovial fluid culture-proved cases of septic arthritis in adults. Thirteen of the patients had well-documented evidence of (1) duration of articular symptoms (presumably reflecting the presence of infection) and (2) subsequent time required to sterilize the synovial fluids (as determined by serial synovial fluid cultures). We found a positive correlation between duration of symptoms and length of time during which serial cultures of synovial fluids remained positive after initiation of treatment. In the seven patients with good outcome, cultures remained positive for 2.4±1.9 days, as opposed to 8.2±2.4 days in the six patients with poor results. The time required to sterilize the synovial fluid appears to be a reliable indicator of response to therapy and an accurate predictor of outcome. Our data show that the rapidity in achieving synovial fluid sterility is of paramount importance in the successful treatment of infection, preservation of articular integrity, and restoration of joint function.