Twenty-two patients with severe pain in the hip on weight-bearing after prosthetic replacement were investigated by radiography, arthrography, scintigraphy and ultrasonography to determine the accuracy of these diagnostic methods in detecting loosening of infective or mechanical origin. In two cases, ultrasonography revealed superficial infections that were resolved with antibiotic therapy. Of the remaining 20 cases, 12 involved a deep infection and in 8 the loosening was of mechanical origin. In the deep infections, standard radiography provided a diagnosis in 66% of cases, arthrography in 75% and scintigraphy in 83%, while ultrasonography detected the deep infection in all 12 cases. In the patients with mechanical mobilization, arthrography and ultrasonography were 100% accurate. Since ultrasonography is able to detect discrete or diffuse lesion of the soft tissues around a prosthesis it should be included among the methods used to screen patients with painful hips after prosthetic replacement.