Purpose: Prevention of late implant bone loss is a critical component in long-term success of implants. The aim of the present study was to evaluate factors affecting late implant bone loss. Materials and Methods: Three hundred thirty-nine endosseous root-form dental implants placed between April 1981 and April 2002 in 69 patients were analyzed. The implants were categorized based on the following factors: (1) surface characteristics (smooth versus rough), (2) length (short [ 10 mm] versus long [¡Ý 10 mm]), width (narrow [ 3.75 mm], regular [3.75 to 4.0 mm], or wide [> 4.0 mm]), (3) the amount of keratinized mucosa ( or ¡Ý 2 mm), (4) location (anterior versus posterior; maxilla versus mandible), (5) type of prosthesis (fixed versus removable), and (6) type of opposing dentition. The effects of these factors on clinical parameters, especially average annual bone loss (ABL), were evaluated clinically and radiographically by a blinded examiner. The parameters evaluated were modified Plaque Index, Gingival Index, modified Bleeding Index, probing depth, and ABL. Results: Shorter implants, wider implants, implants supporting fixed prostheses, and implants in smokers were found to be associated with greater ABL (P .05). The random intercept mixed effects model showed that implant length was the most critical factor for maintenance of ABL. Conclusions: Shorter implants, wider implants, implants supporting fixed prostheses, and implants in smokers were associated with greater ABL. Implant length was the most significant factor in the maintenance of dental implants. Randomized controlled clinical trials are needed to confirm the results obtained from this retrospective clinical study. (Case Series) (More than 50 references.) Keywords: implant maintenance, implant surfaces, late implant bone loss, peri-implantitis