Purpose: This prospective clinical study evaluated the incidence of abutment loosening of Morse taper-connection implants used for single-tooth replacement. In addition, the cumulative survival rate and the implant/crown success were evaluated. Materials and Methods: Implants were evaluated 12, 24, 36, and 48 months after insertion. The incidence of abutment loosening, modified Plaque Index, modified Sulcus Bleeding Index, probing depth, distance from the implant-crown margin to the coronal border of the peri-implant mucosa, width of keratinized mucosa, and the distance between implant shoulder and first bone-implant contact (DIB) were assessed. The cumulative survival rates were calculated with Kaplan-Meier estimates. Implant/crown success criteria included absence of abutment loosening, absence of suppuration and mobility, probing depth 5.0 mm, and DIB 1.5 mm after 12 months and not exceeding 0.2 mm for each following year. Results: Over a 4-year period (2003-2007; mean follow-up per implant: 30.79 months), 307 implants (162 maxillary, 145 mandibular) were inserted in 295 patients (125 men and 170 women aged between 24 and 79 years) at six different clinical centers. The sites included anterior (n = 115) and posterior (n = 192) teeth. At the end of the study, a very low percentage of implant-abutment loosening (0.66%) was found, with only two loosened abutments. The cumulative implant survival rate was 98.4%. Mean DIB was 1.14 mm (48 months). Only four surviving implants did not meet the criteria for success, and the implant/crown success rate was 97.07%. Conclusion: Based upon this study of 307 implants observed during a 4-year period, Morse taper-connection implants represent a good solution for single-tooth restorations, with a very low incidence of abutment loosening (0.66%). Keywords: abutment screw loosening, implant-abutment connection, microgap, Morse taper, single-tooth restorations