The relationship between mobility assessment with the Periotest system and histologic findings was evaluated for oral implants. Five screw-type implants of pure titanium were placed in the mandibles of four monkeys. Two implants in each monkey were occlusally overloaded. These implants were brushed once a week. Plaque was allowed to accumulate around unloaded implants with abutments in the same monkeys. During the experiment, six of eight implants with occlusal overload showed increased manually detectable mobility. Two of these were lost. After 18 months of experimentation, the mobility was assessed using the Periotest system. Sections of the implants and surrounding tissue were cut. For the excessive occlusally loaded implants with manually detectable mobility, positive Periotest values were recorded, and for all other implants the values were negative (range = -7 to -2). All implants with plaque accumulation were histologically osseointegrated but showed marginal bone loss. Two of the implants with occlusal overload had lost osseointegration completely, and two other implants were osseointegrated in the apical part only. A statistically significant association between the Periotest values and the histologic bone level or the proportion of bone-implant contact was observed. If only clinically stable implants (i.e., without manually detectable mobility or with a negative Periotest value) were included in the analysis, no significant correlation was found. The Periotest values revealed only slightly more information concerning the osseointegration of implants than manual mobility assessments.