Purpose: In this experimental study, dental implants placed after maxillary sinus grafting with either porous hydroxyapatite (HA) (Interpore 200) or autogenous bone were examined for their mechanical stress tolerance. Materials and Methods: A total of 54 titanium plasma flame-sprayed cylindric implants were placed in the lateral sinus wall bilaterally of 27 mountain sheep. The bony sinuses were opened through an extraoral approach. Eighteen sinuses were grafted with porous HA, and another 18 were grafted with cancellous bone from the iliac crest. Eighteen non-grafted sinuses were used as controls. In the same operation, 2 cylindric implants were placed in each of the sinuses. One implant of each sinus was tested for mechanical strength of the bone-implant interface at 12, 16, and 26 weeks using pullout force. Results: The mean pullout force was 259.3 N in the control group, 356.7 N in the autogenous bone group, and 376 N in the HA group. Pooled data for the grafted sites showed the pullout force to be significantly higher than in the empty control sites (P = .02). The pullout force increased significantly with ongoing healing time (P = .02), but there was considerable variation within the groups. While the force remained more or less constant throughout the follow-up time in the controls (248 N at week 12 to 276 N at week 26), it increased dramatically in the group augmented with autogenous bone (223.8 N at week 12 to 523.16 N at week 26). The pullout force was initially highest in the HA group (302.3 N at week 12) and increased to 423.5 N at 26 weeks, but it did not reach the levels recorded in the autografted group. Discussion and Conclusion: Mechanical tests of bone-toimplant contact in a sheep model showed that HA for 1-stage sinus floor elevation significantly increased the pullout force versus ungrafted sinuses, although it was less than that found with autogenous bone after 26 weeks.