The effect of systemic deficiency of estrogen on the growth of tissue into porous titanium-fiber implants was studied in ovariectomized Beagle dogs. Five dogs were ovariectomized and five dogs had a sham operation. After waiting four months to allow the levels of circulating estrogen to decline, a titanium-alloy implant that contained four surface pads of titanium-alloy mesh was implanted in the proximal part of the humerus of each dog bilaterally. Two months later, the implants were harvested and subjected to a mechanical push-out test and quantitative histological study. The push-out strength of the implants from the ovariectomized dogs was 31 per cent less than in the control animals. Ovariectomy caused no difference in the amount of ingrowth of bone but resulted in a significant increase in the amount of fibrous connective tissue within the porous pads. The presence of this fibrous tissue appeared to have an important effect on bone-implant fixation: in the control dogs, strength correlated positively with ingrowth of bone and negatively with ingrowth of fibrous tissue, whereas in the ovariectomized dogs, strength correlated positively with ingrowth of fibrous tissue and not at all with ingrowth of bone.