Purpose: To compare clinical outcomes of three-unit fixed prostheses supported by two or three implants in the posterior region. Materials and Methods: Patients in this retrospective study were restored with three-unit fixed partial prostheses supported by two (group 1) or three (group 2) implants in the partially edentulous posterior maxilla and mandible. Outcome measures were survival and success rate, marginal bone loss, complication rate, and peri-implant soft tissue indices. Two-way ANOVA and an independent t test were performed at a significance level of P .05. Result: A total of 48 implants placed in 23 patients (group 1) and 132 implants placed in 40 patients (group 2) were observed for 52.9 ± 19.7 months and 58.5 ± 22.0 months, respectively. Both groups showed a 100% survival rate. The success rate, considering marginal bone loss, was 82% (group 1) and 86% (group 2). Marginal bone loss was greater in group 1 every year during 3 years of measurement, but the difference was not statistically significant (P > .05). Differences of marginal bone loss according to sex, anatomical location (maxilla vs mandible), and connection type (external vs internal) also were not significant (P > .05). Complication rates were similar, but Plaque Index was significantly greater in group 2 (P .05). Conclusion: Within the limitations of this study, three-unit fixed partial prostheses supported by two implants showed comparable clinical outcomes with those supported by three implants in the partially edentulous posterior maxilla and mandible during the short- to medium-term. Keywords: bone loss, implant, soft tissue index, survival rate, three-unit fixed partial prosthesis