Two infants with pyogenic sacroiliitis are presented. In both babies, the clinical findings were initially attributed to septic arthritis of the ipsilateral hip. The correct diagnosis was not established in either infant until radionuclide scanning showed increased uptake in the ipsilateral sacroiliac joints. Both cases emphasize the importance of proceeding quickly to radionuclide imaging in the baby with clinical and laboratory findings of inflammation in the vicinity of the hip and a negative hip aspiration.