7 clinically stable, "osseointegrated", titanium implants, inserted in human jaws for 1-16 years, were retrieved for morphological analysis of the bone-titanium interface, using 3 different preparation techniques. The bone-titanium interface varied as judged from light microscopy of ground sections. The threads of the implants were well filled (79-95%) with dense lamellar bone as quantified with morphometry. A large fraction of the implant surface (56-85%) appeared to be in direct contact with the mineralized bone. In general, the non-bone areas consisted of pockets with osteocytes, bone marrow tissue and/or vessels. Sections were prepared for light microscopy and transmission electron microscopy using a fracture technique, where the implant was separated from the embedded tissue before sectioning, and an electropolishing technique, where the bulk part of the implant was electrochemically removed. In areas judged as direct mineralized bone-titanium contact in the light microscope, the interfacial structure varied at the ultrastructural level. In areas along the interface, unmineralized tissue was present either as a narrow 0.5-1 micron wide zone containing collagen fibril or as deeper pockets containing osteocytes or vessels. In areas with mineralized bone contact, an amorphous granular layer (100-400 nm wide) with no mineral was observed in the innermost interface bordering the mineralized bone, with an electron-dense lamina limitans-like line (approximately 50 nm thick). It is concluded that the bone-titanium interface of the 7 clinically retrieved titanium oral implants examined in the present study bone was heterogenous. In areas of a direct mineralized bone-titanium contact at the ultrastructural level, mineralized bone reached close to the implant surface, but was separated by an amorphous layer, being 100-400 nm thick.