To study the influence of a collagen membrane placed subjacent to the sinus mucosa on the dimensional changes of augmented maxillary sinus floor.Twenty patients were recruited in the study and randomly assigned to two groups. After the elevation of the maxillary sinus mucosa, a collagen membrane with standardized dimensions was placed at the test sites subjacent to the sinus mucosa and the elevated space was filled with a xenograft, both at test and control sites. A collagen membrane was then used to cover the antrostomy at both sites, and sutures were applied to close the wounds. Cone beam computed tomographies (CBCTs) were taken for all patients before surgery (T0), after 1 week from sinus floor augmentation (T1), and after 9 months of healing (T2). Dimensional changes over time of soft and hard tissues were evaluated on the CBCTs.After 1 week of healing, the sinus floor was elevated by 10.0 ± 2.8 mm and 10.6 ± 2.5 mm at the no-membrane and membrane groups, respectively. After 9 months of healing, a similar reduction of the height was observed in both groups, providing a total vertical augmentation of 8.6 ± 2.8 mm at the no-membrane sites and 9.1 ± 3.1 mm at the membrane sites. After 9 months of healing, the hard tissues subjacent to the sinus mucosa appeared to be partially corticalized in three patients in the no-membrane group and in six patients in the membrane group.The use of collagen membranes subjacent to the sinus mucosa did not influence the dimensional variations of the augmented regions and the clinical outcomes after 9 months of healing also in absence of perforations.