Infected olecranon, prepatellar, and infrapatellar bursae offer a unique opportunity to study the response of a closed-space infection to antibiotic therapy. Using percutaneous needle aspirations, serial bursal fluids were cultured. The length of time necessary to achieve culture sterility with antibiotic therapy (an average of 4 days in 25 patients) was correlated with the duration of symptoms prior to diagnosis (r = 0.68, P less than 0.001). In patients treated within 2 weeks from onset of symptoms, bursal fluid sterility was achieved within 1 week of therapy, while longer duration of symptoms was associated with delayed response. When antibiotic therapy was continued for 5 additional days after documented culture sterility, all 19 patients in the prospective trial were cured (average followup period of 6.8 months). In septic bursitis, the effects of delay in treatment are deleterious by prolonging culture-positivity despite adequate antibiotic therapy. By analogy, delay in treatment of septic arthritis may result in the persistence of an adverse environment which can lead to further articular damage. After accurate diagnosis of septic bursitis, a therapeutic approach consisting of prompt and appropriate antibiotic usage, frequent needle drainage, and treatment duration based on the culture results of serial aspirations is effective and may be applicable in the management of certain nongonococcal bacterial joint infections.