In many countries Haemophilus influenzae type b (Hib) is the second most common cause of septic arthritis in children. In Finland large-scale immunisation against Hib using conjugate vaccines began in 1986, four years after a multicentre prospective study of orthopaedic infections in children had started. Since 1982, including six years before and ten after starting routine Hib vaccination, there has been a major change in the pattern of septic arthritis. From 1982 to 1988, 32 of 61 cases (53%) were caused by staphylococci, 22 (36%) by Hib and 7 (11%) by other bacteria. Since 1988, Hib infection has disappeared, and one-third of cases of childhood septic arthritis has been eliminated. This change has allowed us to reduce initial antimicrobial therapy for such children to cover only Gram-positive cocci. The more limited treatment is safer, reduces cost, and simplifies treatment