Septic arthritis results from invasion of the synovium by microorganisms, and most infectious organisms can cause arthritis. In children, septic arthritis is usually associated with systemic infections such as pneumonia or meningitis; in young adults, gonococcal arthritis is the most common cause; and in the aged, septic arthritis is usually superimposed on chronic joint disease. Underlying rheumatoid arthritis is common, and Dr. Knights stresses the importance of recognizing septic arthritis as a complication of rheumatoid arthritis.