The goal of this study was to histologically document the effect of two different implant surfaces on the percentage of bone-to-implant apposition achieved with implants placed in human sinus grafts. The influences of implant site and postgrafting delay time were also examined. Nine healthy volunteers were scheduled to undergo posterior maxillary sinus floor augmentation in preparation for delayed implant placement. In addition to the conventional dental implants selected for each case, titanium alloy experimental implants, 2.5 mm in diameter and 8 mm in length, were custom manufactured. Each microimplant was prepared longitudinally with two different surface topographies: machined on one side and MTX microtextured on the other side. A notch prepared across the superior aspect of the implants facilitated placement and provided a reference line between the two surfaces. Patients were divided into two groups that received the experimental microimplants at the time of conventional implant placement: Group A (six patients) received the experimental microimplants in the regenerated lateral wall of the sinus graft 11 months after graft placement, and group B (three patients) received them in the alveolar crest 6 months after graft placement. After 6 months of submerged healing, all experimental and conventional implants appeared to be clinically osseointegrated. Histologic analysis revealed that the mean bone-to-implant apposition was significantly greater with MTX (72.31% ± 17.76%) compared to machined surfaces (38.01% ± 19.32%), regardless of bone quality. The healing time between graft and implant placement and implant location did not statistically impact the percentage of bone-toimplant apposition.