Implants used to replace molar teeth present issues of space distribution and crown contours that are unique to these sites. Because the diameter of the implant typically is smaller than that of the tooth being replaced, and the circular shape of the implant does not replicate the anatomy of the tooth, a large gingival embrasure area between the implant platform and adjacent tooth often results. Upon the review of radiographs of molar implants placed over an 11-year period in one private practice, the authors identified an unusually high incidence of approximal, cervical, and root caries on teeth adjacent to these implants. This retrospective radiographic study investigated the incidence of decay as related to: (1) the horizontal distance from the implant to the adjacent tooth (ie, the implant-tooth distance, or ITD) as measured at the alveolar crest, (2) the vertical distance from the apical portion of the prosthetic contact area to the implant platform, and (3) the presence of an existing restoration on the adjacent tooth. Of the three variables examined, both the horizontal distance of the implant to a natural tooth (ITD) and the presence of an existing restoration on an adjacent tooth proved to be significantly correlated with the incidence of decay. Results showed that incidence of decay ranges from 7.4% when the ITD is <2 mm to 40% when the ITD is ≥6 mm. The mean ITD in cases in which decay was found was 4.1 mm, and it was 3.5 mm in cases that showed no decay (P = .005). It is therefore suggested that the horizontal threshold of 4 mm be considered as the "critical ITD." The results also indicated that the presence of a previous restoration on an adjacent tooth increases the incidence of decay to a statistically significant degree with an odds ratio of 2.25 at a 95% confidence level. This information may prove useful in diagnosis and treatment planning for molar implant replacement.