Data from 1158 cases of septic arthritis reported to the Public Health Laboratory Service (PHLS) Communicable Disease Control Centre (CDSC) from England and Wales over a 4 yr period (January 1990 December 1993) are presented. Reports where a bacterial organism was isolated from synovial fluid, or where an organism was isolated from blood cultures where a diagnosis of septic arthritis was reported, were examined. Reports of infection were more common in children (12.7% of infections were in the under 10 age group) and the elderly (54.7% aged 60 or over), and were higher in males in all age groups except in the elderly. The most common causative organisms remain staphylococcal and streptococcal species, comprising 40.6% (470) and 28% (324) of cases, respectively. The most common streptococci seen were Streptococcus pneumoniae and Lancefield group A beta-haemolytic Streptococcus organisms, 60.8% (197/324), although group B, C and G organisms accounted for 33.6% of streptococcal isolates (109/324). Haemophilus influenzae septic arthritis is not exclusive to children as 23.2% (16-69) of cases occurred over the age of 15. A total of 48% (635) of isolates were identified from both synovial fluid and blood cultures, 32.6% (378) from joint fluid alone and 12.5% (146) from blood cultures. Although this study excludes cases of septic arthritis where no organism was isolated, it presents important bacteriological information from a large number of isolates from England and Wales over a 4 yr period. Risk factors identified include a joint prosthesis, joint disease/connective tissue disorder. immunosuppression and diabetes.