"Aim: The aim of this study was to evaluate the reasons for implant failure in two Oral and Maxillofacial Units in Helsinki. Seventeen patients, who lost 30 implants were included in this study. Material and methods: The patients and implants were studied clinically, radiologically and microbiologically. Results: Most patients did not have any symptom indicating failure; hence, the failures were noticed mainly by the clinicians when instability of the fixture or of the prosthetic reconstruction became obvious. Radiolucency around the fixture was the most frequent radiological finding. Twenty per cent of the fixtures were located in insufficient bone. Ninety seven per cent of the bacterial cultures were positive, Streptococcus milleri being the most commonly identified aerobic and Fusobacterium nucleatum the most commonly anaerobic bacteria. Conclusion: The most critical time for success is immediately after prosthetic loading. Hence, implants should be placed in the optimal position to facilitate prosthetic reconstruction and loading. © 2005 European Association for Cranio-Maxillofacial Surgery."