We conducted a retrospective study comparing the presenting clinical and biologic features of 64 children who had septic arthritis caused by Kingella kingae with 26 children who had septic arthritis caused by Staphylococcus aureus. Children with K. kingae septic arthritis were significantly younger than those with S. aureus septic arthritis. Otherwise, there were no significant differences between the 2 groups with respect to fever, location, white blood cell count, synovial fluid cell count, C-reactive protein, or serum fibrinogen. However, the clinical course was significantly better for children with septic arthritis caused by K. kingae as evidenced by shorter hospitalization and fewer adverse events. Presumptive antibiotic therapy for septic arthritis in young infants should take into account both of these pathogens, even in case of mild presentation.