Septic arthritis is most often monoarticular but may affect more than one joint. Bacteria responsible for joint infection vary with age group, similar to the organisms producing meningitis. Haemophilus influenzae, Staphylococcus aureus, and gram-negative bacteria predominate in early childhood; S. aureus, pneumococci, and streptococci in later childhood; and S. aureus, pneumococci, streptococci, and gonococci in adults. Conditions that predispose to gram-negative bacilli include neoplasia, immunosuppression or decreased immunologic defenses, intravenous drug addiction, and urinary tract infections. Septic arthritis is diagnosed by direct aspiration and culture of the synovial fluid. Gram stain is reported to be positive in 40%-75% (range, 30%-95%) of infected joint aspirates, with detection of gram-positive aerobic organisms accomplished more readily than gram-negative organisms.