Undecalcified hard-section histology was performed on twelve canine and eleven human porous-coated femoral hip prostheses implanted without bone cement. The period of implantation varied from four weeks to seven years. All of the canine and nine of the eleven human implants showed regions of substantial bone ingrowth along the porous-coated stem region. Areas of the two human implants ingrown with fibrous tissue were characterized radiographically by a radiopaque line surrounding, but separated from, the implant surface. Resorptive bone modeling was observed with both implant types, particularly proximally. New bone formation and hypertrophy typically occur at regions of proximity to or contact with the endosteal cortex and, in the case of proximally porous coated implants, at the junction of the porous and smooth implant surfaces. The smooth-surfaced distal region of proximally coated implants is typically surrounded by a thin shell of bone that is separated from the implant surface. Overall, resorptive and formative bony change appears more exaggerated in the canine, but the radiographic and histologic pictures of canine and human implants bear resemblances.