Clinical studies of mini-dental implants (MDIs) used for retention of removable partial dentures (RPDs) in patients without posterior teeth have not been reported yet. The aim of this thesis was to prospectively explore whether MDIs can be used for retention of RPDs in Kennedy Class I and Class II patients without posterior teeth with narrow alveolar ridges (≤4.5 mm). MDIs were inserted without raising a flap in previous canine or first premolar sites. All studied patients received the new RPDs. The MDIs were early or late loaded. Marginal bone loss (MBL) was measured on the follow-up intraoral radiographs. The MDI survival- and success rates, the Modified Plaque Index (MPI), and the modified Bleeding Index (MBI) were assessed. The dental patient-reported outcome measures (dPROMs) were recorded using: the Orofacial Esthetic Scale (OES), the 14-item Oral Health Impact Profile (OHIP-14), and the Chewing Function Questionnaire (CFQ). All technical complications with RPDs during the observation period have been registered. Statistical analysis comprised sample size calculation, descriptive statistics, Kolmogorov-Smirnov test, X2 test, Student’s ttests, one-way ANOVA, ANCOVA, repeated measures, effect size calculation, and survival curves. A total of 84 Kennedy Class I patients were included (78.6% females; mean age 66.0±7.6 years). Of this, 57 received MDIs in the mandible and 27 in the maxilla. During the first year, four MDIs were lost shortly after loading. Two MDIs were lost in the maxilla, two in the mandible (in four different patients). None of the MDIs was lost afterward. At the implant level (in 81 patients) both, the success and the survival rates were 95.3% after one year. At the 2-year recall, the survival rate was 93.4%, and the success rate 91.8% (assessed in 61 patients). The MBI and MPI medians equaled one; however, oral hygiene worsened significantly during the 2nd year. Mean 1st year MBL was 0.23±0.35 mm and 0.12±0.18 mm during the 2nd year, but it raised to 0.30±0.47 mm after two years. Only age had significant effect on MBL, whereas gender, the target jaw for implant insertion, and the dental status of the antagonistic jaw did not influence the MBI. All dPROMs showed large effect size, which remained unchanged throughout the two years. Kennedy class II patients (11 females, three males, 52-80 years old, 8 MDIs in the mandible, 6 in the maxilla) showed similar rates of MBL as Kennedy Class I patients, and none of the MDIs were lost. Within the limitations of this study, the MDI-RPD was a successful treatment option in Kennedy Class I and II patients, with narrow ridges, at least in the first two years after implant-prosthodontic rehabilitation.