We have previously studied a large number of histological specimens of biomaterials and found that regions with and without an intervening fibrous membrane coexisted in many specimens. Therefore, it appears necessary to perform an evaluation of the entire specimen when histologically assessing the affinity of bone for a biomaterial. Accordingly, we performed a quantitative histological evaluation of hydroxyapatite (HAP)- and titanium-oxide(TiO2)-coated Ti-6A1-4V and uncoated Ti-6A1-4V (control) by determining the affinity index. This was defined as the length of bone directly opposed to the implant/the total length of the bone-implant interface X 100%. The test materials were inserted into the distal epiphyseal region of the femurs of adult dogs, and follow-up quantitative histological comparisons were performed from 4 weeks to 96 weeks. The HAP-coated implants had the highest affinity index 4 weeks after insertion, and this superiority was maintained up to 96 weeks. There was a significant difference in affinity index between HAP-coated implants and control implants (P less than 0.001), while TiO2-coated implants showed no significant difference in comparison to the control.