Ultrasound scans of the hip were carried out in 132 children with hip pain during an 18-month period to evaluate the hip for the presence of an effusion. Seventy-three of these patients were followed up long enough to ascertain the presence or absence of septic arthritis. The remaining 59 patients were discharged with diagnoses other than septic arthritis but could not be located to confirm the ultimate accuracy of the diagnosis. Four patients were initially determined to have no effusion but subsequently were diagnosed with septic arthritis (false-negative rate of 5%). Two of these patients had inadequate initial ultrasound examinations. Two children had ultrasound examinations that even on retrospective review did not reveal an effusion. Both of these children had had symptoms for <24 hours, and one had a contralateral hip effusion. The authors recommend using the negative results of an ultrasound scan as evidence of the absence of septic arthritis in children with caution when symptoms have been present for <24 hours or when bilateral disease exists.