AbstractThis study is the first description of the extensive porosity which is preferentially located at the cement‐prosthesis interface of cemented femoral components of total hip replacements. The observation is important because the interfacial porosity may decrease the strength of the cement‐femoral prosthesis interface and jeopardize the mechanical integrity of the cement mantle. We examined the cement‐metal interfaces from a multiplicity ofin vivoandin vitrospecimens using both optical and scanning electron microscopy. These samples included several stem designs, implants made from either Co‐Cr or Ti alloy, implants made with a variety of surface finishes and both centrifuged and uncentrifuged cement. Allin vivoandin vitrosamples had marked porosity in the cement focally concentrated at the cement‐metal interface. The amount of porosity at the interface greatly exceeded the amount of general porosity found throughout the bulk cement. Centrifuging did not affect the interfacial porosity, and neither did alloy nor surface finish. The presence of these pores may be expoained by the rheological characteristics of the cement. © 1993 John Wiley