The aim of the present study was to compare the clinical, radiological and histomorphometrical outcome of simultaneous implant placement following augmentation of atrophic maxillary sinuses using allograft (block or particles). Consecutive patients with maxillary residual alveolar ridge height ≤3 mm, scheduled for sinus floor augmentation with simultaneous implant placement, were randomly included. Allograft bone-block or bone-particles served as grafting material. Simultaneously, dental implants were inserted. Biopsies were taken at second stage surgery (after 9 months) for histomorphometric evaluation. Initially 38 sinus augmentations (29 individuals) were allocated for the study. In 4 out of 21(19%) sinuses using particles it was impossible to stabilize the implants and a second stage insertion was preferred, leaving 34 sinuses for histomorphometric evaluation. The difference in the ability to perform simultaneous implant placement was statistically significant (p < 0.05). Ninety implants were inserted simultaneously. All implants osseointegrated. None of the implants was lost up to the end of follow-up time (Range 50–120 months, Mean 74.5 ± 13.5 months). Bone gain radiographically 12.3 ± 1 mm vs. 11.2 ± 1 mm (block vs. particles respectively) and new bone formation histomorphometrically 27.7 ± 15% vs. 32.1 ± 19% (block vs. particles respectively) showed no statistically significant differences between the two groups. Sinus augmentation using allograft (particles or block) and simultaneous implant placement is predictable. All outcome parameters are similar when sinus bone-blocks augmentation is compared to bone-particles augmentation (radiological new bone gain, implant survival, hisomorphometricly new bone formation) despite the ability to stabilize implants, when placed simultaneously with sinus augmentation. Blocks may be advisable when simultaneous implant placement is imperative in cases with residual alveolar bone height ≤3 mm.