Purpose: To compare the efficacy of two different techniques to augment maxillary sinuses using a lateral window approach: rigid synthetic resorbable barriers (Inion) versus granular anorganic bovine bone (Bio-Oss). Materials and methods: Ten partially edentulous patients having bilaterally 1 to 5 mm of residual bone height and at least 5 mm bone width below the maxillary sinuses, as measured on computed tomography (CT) scans, were randomised to receive two different 2-stage sinus lift procedures using the lateral window approach. In one side, the sinus lining was raised by placing a resorbable rigid Inion barrier without any bone substitute whereas the contralateral side was loosely packed with 100% granular Bio-Oss. After 6 months, 2 to 3 implants were inserted at each side and submerged for 4 months. Implants were loaded with provisional acrylic prostheses and replaced after 4 months, by definitive screwretained metal-ceramic prostheses. Outcome measures were: time necessary to complete the augmentation procedure, bone gain on CT scans, histomorphometry, any complication, implant and prosthetic failures, and clinician and patient preference assessed by a blinded outcome assessor. All patients were followed up to 5 months after loading. Results: No patient dropped out. There was no significant difference in time to complete the augmentation procedure (19.8 minutes for Inion versus 20.5 for Bio-Oss). After 6 months, both interventions gained bone in a highly statistically significant way (14.4 mm for Inion versus 14.1 mm for Bio- Oss) with no significant differences between the procedures. Histologically, more new bone formed at Bio-Oss treated sites (36.1% versus 24.2%), the difference being highly statistically significant (P = 0.002). There were no differences in complications between groups (2 perforations of the maxillary lining at Inion treated sites versus 1 at a Bio-Oss site), however, in one of the patients where a perforation occurred at the Inion site, at implant placement, the sinus was two-thirds filled with soft tissue and the site was successfully retreated with Bio-Oss. No implant failed. The clinician preferred Bio-Oss because it was simpler to handle. There were no statistically significant differences in patient preference 1 month after surgery and 1 month after delivery of definitive prostheses: 8 patients had no preference while 2 preferred the Bio-Oss treated side. Conclusions: Although bone grafting is not needed to augment atrophic maxillary sinuses since it is sufficient to keep space with a rigid barrier, bone was histologically more mature and appeared to be clinically harder when using Bio-Oss. Moreover, it was judged simpler to fill sinuses with a bone substitute than to position a rigid barrier for maintaining space. Keywords: anorganic bovine bone, resorbable barrier, sinus lift