Dental implants with a reduced diameter are commonly used where bone width is narrow or in cases of restricted mesiodistal anatomy such as laterally maxillary and mandibular incisors. They could also be a viable alternative to bone augmentation especially in challenging situations such as the posterior regions of the mandible. While it has been shown that it is possible to horizontally augment bone in mandible with different procedures, these techniques are associated with significant postoperative morbidity and complications, can be expensive and technique sensitive, and require long treatment periods. Narrow-diameter implants could be simpler, cheaper, and faster alternative to horizontal bone augmentation in the mandible, if they will provide similar success rates. This cohort study was designed to evaluate whether NDIs (2.75 and 3.25 mm diameter) could be used to support partially fixed prostheses in posterior mandibles having insufficient bone ridge thickness for placing standard-diameter implants. At 1-year post loading, implant survival rate was 97.6%, the number of complications was low, and the implants lost an average of 0.47 mm of peri-implant bone. The present data are similar to those observed around other implant systems used in the similar condition. Malo et al. [6] reported a 95.1% survival rate after 11 years of function for narrow-diameter implants (3.3 mm diameter) placed in posterior regions of both jaws. The values for marginal bone resorption recorded in this study at 1, 5, and 10 years (not exceeding 0.2 mm/year of bone loss after the first year) are within the accepted standard success criteria for implants. Regarding the implant failures, the majority occurred in the first 6 months of function, following the pattern for standard-diameter implants. In another retrospective study, Anitua et al. [10] observed a survival rate of 97.3% for 2.5 mm diameter implants used as definitive implants for rehabilitation of missing teeth having a follow-up between 3 and 7 years.