Acrylic bone cement is commonly used to affix metallic femoral hip implants within the proximal femur during hip arthroplasty. Previous studies indicate that the expected life of a cemented implant may depend on the integrity of the metal-cement interface and that the interface strength can be influenced by many factors such as variable cement technique, surface roughness of the metal implant, or using pre-coated implants. To this date, there is no study, as far as is known, on the effect of cement thickness on the integrity of the metal-cement interface. The purpose of this study was to determine whether different cement mantle thickness affected the mechanical integrity of the metal-cement interface. Two different surface roughness of commercially manufactured Co-Cr cylindrical implants were cemented into plastic cylindrical molds simulating the femoral intramedullary cavity. Post-cured specimens were evaluated under four test conditions including 24-hours stored dry at room temperature (rt), 3-months dry (rt) storage, and 3-months immersion in physiological saline solution (rt) or at body temperature (bt). Parametric analysis of the metal-cement interface, measured with the push-out test method, indicated that all grit-blasted implants regardless of cement thickness and test conditions had mean interface strength values of approximately 5 MPa. With the polished implants and thick cement mantle (i.e. 6.3 mm) the mean interface strength was an order of magnitude lower, i.e. ≈ 0.5 MPa, and was significantly reduced after 30 days of immersion. For the polished implants with thin cement mantle (i.e. 2.4 mm) the mean interface strength values were approximately 0.1 MPa for all test conditions. In summary, in this study the metal-cement interface bond was maintained with grit-blasted surfaced implants under the different test conditions; however, debonding of polished surfaced implants with thick cement mantle was sensitive to test conditions and that debonding was readily obtained with the thin cement mantle. It is concluded that thin cement mantles around metallic femoral stems may result in earlier debonding at the metal-cement interface thereby increasing the risk of early failure of a cemented implant system with cylindrical stems.