The clinical and microbiologic features of septic arthritis in 23 elderly patients are reviewed. Fifteen patients had pre-existing joint diseases, predominantly osteoarthritis and rheumatoid arthritis. Eight patients had underlying systemic illnesses, and eight patients were receiving systemic corticosteroid therapy prior to the development of septic arthritis. The knee was the joint most commonly infected. Although Staphylococcus aureus was the major pathogen (52.2 per cent of patients), enteric gram-negative bacilli were found in seven of 23 patients (30.4 per cent). Five patients died (21.7 per cent mortality), two as a result of their infection and three of nosocomial Pseudomonas sepsis. Eight of the 18 survivors (44.4 per cent) developed osteomyelitis in the contiguous bone. Return of joint function was slow in all patients. Septic arthritis in the elderly is difficult to treat and has a poor outcome, possibly because pre-existing joint disease is very common and enteric gram-negative bacilli are often the causative organisms.