At all ages, Staphylococcus aureus is the most common microorganism responsible for septic arthritis, but in children below five years of age Haemophilus influenzae is frequently found. In about 1/3 of the cases cultures are negative, primarily because of initiation of antibiotic therapy prior to sampling. A distended joint should be treated as infection until otherwise proved. Today repeated aspirations of the infected joint are recommended as initial treatment, if possible with additional joint lavage, together with systemic antibiotic therapy. In case with no clinical response within 2-4 days, surgical debridement of the joint with synovectomy is of utmost importance. Exercises without weight-bearing are initiated at an early stage and weight-bearing avoided for six weeks.