Purpose: This experimental study in adult female sheep examined the value of nonresorbable porous hydroxyapatite (HA) as a grafting material in a single-stage sinus-lift procedure. Materials and Methods: Two titanium plasma-flame-sprayed cylindric implants were placed bilaterally in each of 54 sinuses in 27 adult female sheep. In 2 groups of 18 sinuses each, the subantral hollow space was filled with porous HA or autogenous cancellous bone harvested from the iliac crest, respectively. Eighteen sinuses were not augmented and served as controls. The time course of new bone formation and bone remodeling was evaluated by sequential polyfluorochrome labeling. Observation periods were 12, 16, and 26 weeks after the surgical procedure. Six sinuses per observation period and test group were available for histologic evaluation. Results: All implants were osseointegrated in the local host bone. New bone formation was observed in a triangular area bounded by the implant surface, local buccal antral wall, and submucous connective tissue around all implants. The mean length of bone-implant contact was 3.9 ± 0.3 mm in the control group, 5.7 ± 0.3 mm in the autogenous bone group, and 5.9 ± 0.3 mm in the group augmented with porous HA. During the observation period, the relative length of direct boneimplant contact increased from 20% to 25.1% in the control group, from 30.4% to 35.5% in the autogenous bone group, and from 29.8% to 41.7% in the HA group. At a distance of 1 mm from the implant, the mean bone volume was 29.7 ± 15.7% in the autogenous bone group. In the group augmented with HA the mean bone volume was 11.2 ± 13.0%. Discussion: There was no significant difference between HA and autogenous bone regarding bone-implant contact (P = .89). Conclusions: Both groups showed a significantly greater bone-implant contact (HA: P = .002; autogenous bone: P = .0005) than the empty control group. However, since the results varied widely, the use of HA alone for sinus grafting should be used with discretion in sinus-lift procedures.