Purpose: To compare the clinical outcome of 6-mm and 10-mm long implants in partially edentulous posterior areas. Materials and methods: Twenty-four patients, with a partially edentulous area in the jaws with a height and width allowing the positioning of 2 to 3 adjacent 10 × 4.1 mm implants without any augmentation procedure, were randomly allocated according to a parallel group design to receive 6-mm long or 10-mm long implants. A total of 54 implants were placed (26 × 6 mm and 28 × 10 mm implants). Patients were restored 8 weeks after surgery and were followed for 5 years. Outcome measures were prosthesis and implant survival, as well as marginal bone level changes and complications. Results: After 5 years, 18 patients were available. One 6 mm implant failed during the healing period and its related prosthesis could not be placed. No implants were lost after loading. The 6 mm group registered 5 complications (1 mucositis, 3 prosthesis decementations and 1 chipping), while only 3 were registered in the 10 mm group (2 decementations and 1 chipping). The difference in complications between the two groups was not statistically significant (P = 0.39). Marginal bone loss at 5 years was 0.43 and 0.24 mm with the 6 mm and 10 mm groups, respectively (not statistically significant; difference between the two groups 0.19 mm; SD 0.23 mm; 95% CI -0.34;0.73; t test P = 0.42) Conclusions: Implant and prosthetic survival and success rates were similar between prostheses supported by 6-mm or 10-mm long implants.