Purpose: To assess the pattern of bone loss in peri-implantitis in partially edentulous patients and relate patient and implant/site characteristics that may influence such patterns. Materials and Methods: In this retrospective study, the records of partially edentulous patients with dental implants were stratified according to sex, age, smoking, and diabetes. Implants were stratified according to site, number of years in function, presence of adjacent implants, diameter, bone-grafted site, and implant platform. All these variables were obtained from previous periodontal charts and radiographs. The implants were then classified into two main groups: (1) bone loss as defined by the presence of progressive marginal bone change found on radiographs at least 12 months following prosthesis placement compared to a baseline with a threshold ≥ 2 threads of bone loss; and (2) no bone loss as defined by no detected bone change or bone level change with a threshold 2 threads. The bone loss group was further divided into three subgroups according to pattern: vertical, horizontal, and combined. Descriptive analyses were applied to assess the frequency of the pattern of bone loss (horizontal, vertical, and combined). A statistical regression model was used to find if there was a significant correlation between patient/implant characteristics and the pattern of bone loss. Results: A total of 304 charts with 540 implants met the inclusion criteria. One randomly selected implant per patient through Microsoft Excel software was included in this study. Of these, 157 (51.6%) of examined charts were men and 147 (48.4%) were women. The patients' mean age was 63.9 ± 11.4 years (range: 27 to 85 years) at implant placement, and implants had been in function for 12 to 120 months (median: 37 months). The percentage of implants that had bone loss was 24.7% (75 out of 304). The pattern of bone loss was 65%, 22%, and 13% for vertical, horizontal, and combined, respectively. Implants that had been placed in surgically bone-grafted sites had increased odds of vertical bone loss with either narrow or wide implants (OR = 2.5 [P = .04] and 3.1 [P = .01], respectively). The presence of adjacent implants had significantly (P = .003) increased odds of horizontal bone loss (OR = 5.1). Conclusion: Approximately one-quarter of dental implants (24.7%) developed bone loss beyond normal physiologic remodeling. Vertical bone loss around single implants was the most common pattern (65%), particularly around implants placed in bone-grafted sites with odds ratio of 2.5 for narrow implants vs 3.1 for wide implants. In the presence of adjacent implants, the odds of horizontal bone loss was 5.1 (P = .003). Keywords: dental implant, pattern of bone loss, peri-implantitis, risk factors