PURPOSE: The purpose of this study was to determine the efficacy of sonography in the evaluation of normal pseudocapsular morphology and the detection of complications after total hip arthroplasty. MATERIALS AND METHODS: Between Janvary 1997 and June 2000, 47 patients [35 men and 12 women aged 24 to 84 (mean, 61) years] using real-time linear-array, convex US units with 3.5-MHz and 10-MHz transducers. Normal capsular morphology in 30 with total hip replacements, who had been asymptomatic for at least one year, was studied, and the prosthetic joint infection demonstrated in six of 17 who had experienced was confirmed at surgery or by US-guided aspiration. RESULTS: Sonograms indicated that a normal pseudocapsule lay straight over the neck of the prosthesis or was slightly convex toward the neck, and that the mean bone-to-pseudocapsule distance was 2.9 mm. However, in the 11 symptomatic patients in whom no evidence of infection was revealed by cultures, th mean distance was 4.7 mm; in the remaining six patients, whose joints were infected (a condition strongly suggested by the presence of extracapsular fluid), the mean distance was 5.5 mm, with no significant difference between the two groups. CONCLUSION: Sonography can be used to evaluate normal caspular morphology after total hip replacement and to diagnose infection around hip prostheses. In all patients in whom sonography revealed the presence of extra-articular fiuid, infection had occurred.