Thirty patients with a painful hip arthroplasty had an In-111 leucocyte scan before surgical reexploration. In 12 patients, the In-111 leucocyte scan was abnormal and in all of them, microorganisms were found at the culture of the material from their hips at the operation. Among the 18 patients with a normal scan no infection was found in 17. In one patient, a thick-walled abscess growing Escherichia coli was found. We conclude that In-111 scanning is sensitive, specific and therefore useful in the differential diagnosis of pain after hip arthroplasty.