The authors described 17 patients who had had resection arthroplasty of the hip and who were suspected of having an infection. The resection arthroplasties had been performed for previous infection. All the patients were studied by ultrasonography to detect effusion in the pseudoarticular space. Thirteen of the 17 patients were found to have an effusion by ultrasonography. Fluid was obtained in 9 of the 13 patients by ultrasonographic-guided aspiration. The mean aspirated volume was 3 ml (range 1-25 ml). Five of the 9 aspirates proved to be septic. The echopattern in all but one of those five with sepsis was nonechofree. Of four other patients in whom it was not possible to aspirate fluid lavage of the pseudoarticular space one yielded a positive culture. The role of ultrasonography in the diagnosis and management of patients who have undergone resection arthroplasty of the hip and who are suspected of having an infected pseudoarticular space is discussed.