Any patient who presents with an acute monarticular arthritis, especially a new asymmetric effusion with underlying joint disease, should be suspected of having a bacterial process. Because synovial fluid findings (leukocyte counts and glucose) may not be predictive of infection, bacteriologic analysis by smear and culture is necessary in the evaluation of any new synovial effusion. A chronic monarticular process is highly likely to be infectious also, but mycobacterial or fungal etiologies frequently require appropriate culture of synovial tissue in addition to processing fluid. Acute polyarticular syndromes are seen as manifestations of disseminated gonococcal infections (DGI) and certain viral infections in adults. Diagnostic clues include historic and physical findings (exposure history and type of rash). The major pathogen in adults remains Staphylococcus aureus, so initial therapy is directed at this organism unless urinary tract infection is present also. Proper recommended therapy for DGI is ceftriaxone because penicillin-resistant strains are present in many urban centers. Early recognition and treatment of bacterial arthritis may prevent poor outcome, particularly in elderly patients or those with underlying joint diseases. For chronic mycobacterial or fungal infections, surgery may need to be combined with medical management.