Septic hip is rare in patients with femoral head osteonecrosis (FHON) and has been known to occur in those who are immune compromised. Thus, surgeons do not concern about the superimposed septic hip in immunocompetent patients. We report five patients who had FHON and concomitant septic arthritis of the hip. No patients were immune compromised. All patients had an elevation of ESR and CRP. Two patients had an otherwise unexplained fever preoperatively. Two patients had had a remote septic focus. When a patient with FHON has an unexplained elevation of ESR and CRP, concomitant septic arthritis of the hip should be suspected even though the patient is not immune compromised.