Serial radiographs of 101 total hip arthroplasties were reviewed to evaluate the clinical significance of lucent zones at the femoral metal-cement interface. Such lucent zones were identified in 19 hip prostheses, but only in four was there clinical evidence of infection loosening of the prosthesis. The remaining 14 patients (15 prostheses) were followed for periods ranging from 18 months to 5 years; none had complications despite the development of these lucent zones. Microdensitometry confirmed the existence of a true lucent zone in most cases, but in several the apparent lucency was due to the Mach effect. While progressive widening of the lucent zone or a lucency measuring greater than 2 mm in width should prompt careful clinical evaluation for evidence of infection or loosening, our data indicate that such complications need not necessarily be present.