Prompt diagnosis and treatment of infectious arthritis can help prevent significant morbidity and mortality. The acute onset of monoarticular joint pain, erythema, heat, and immobility should raise suspicion of sepsis. Constitutional symptoms such as fever, chills, and rigors are poorly sensitive for septic arthritis. In the absence of peripheral leukopenia or prosthetic joint replacement, synovial fluid white blood cell count in patients with septic arthritis is usually greater than 50,000 per mm3. Isolation of the causative agent through synovial fluid culture is not only definitive but also essential before selecting antibiotic therapy. Synovial fluid analysis is also useful to help distinguish crystal arthropathy from infectious arthritis, although the two occasionally coexist. Almost any microorganism can be pathogenic in septic arthritis; however, septic arthritis is caused by nongonococcal pathogens (most commonly Staphylococcus species) in more than 80 percent of patients. Gram stain results should guide initial antibiotic choice. Vancomycin can be used for gram-positive cocci, ceftriaxone for gram-negative cocci, and ceftazidime for gram-negative rods. If the Gram stain is negative, but there is strong clinical suspicion for bacterial arthritis, treatment with vancomycin plus ceftazidime or an aminoglycoside is appropriate. Evacuation of purulent material with arthrocentesis or surgical methods is necessary. Special consideration should be given to patients with prosthetic joint infection. In this population, the intraarticular cutoff values for infection may be as low as 1,100 white blood cells per mm3 with a neutrophil differential of greater than 64 percent.