In this in vitro study we evaluated the initial stability of cementless femoral stems using an injectable calcium phosphate (Ca-P) cement. The cement was not used to form a cement mantle as is routinely done in PMMA cemented prostheses but functioned as an additive to fill the small gaps that exist between a press-fit placed titanium plasma sprayed implant and the bone bed. Six pair of Beagle femora were used in this study. In a random fashion, one femur of each pair was used for placement of a prosthesis without Ca-P cement, the contralateral was used for press-fit placement after injection of the calcium phosphate cement into the intramedullary canal. The reconstructions were placed in a MTS testing machine, tilted 15 degrees in varsus and 15 degrees of endorotation to obtain a physiological load on the femoral head. The load was applied stepwise from zero to a maximum of 100, 250 and 400 N, respectively. At each loading step the load was applied dynamically at a frequency of 1 Hz for 30 min. Between the loading steps, the load was removed for 10 min to allow elastic recovery. The stability of the stems was determined at each loading step with roentgen-stereophotogrammetric analysis. Results showed that with the prostheses without Ca-P cement the most important displacements were movement into varus (max. 818 microm under 400 N) and subsidence (max. 587 microm under 400 N). The displacements showed large variation. After unloading some elastic recovery occurred. In the specimens with Ca-P cement, displacements were negligible. As determined by an F-test the variations found were significantly smaller for the press-fit+Ca-P cement relative to the press-fit prosthesis at all loading steps (p<0.05). A paired t-test revealed significant differences in the mentioned displacements between the press-fit- and press-fit+Ca-P cement prosthesis at a loading with 400 N (P<0.05). On the basis of these results we conclude that the use of Ca-P cement increases the initial stability of press-fit inserted plasma-sprayed femoral prostheses and corrects for the high variability in displacements found with press-fit insertion of these femoral hip prostheses.