Purpose: The purpose of this study was to evaluate the clinical effectiveness of hydroxyapatite-coated cylindrical implants to support overdentures. These implants were placed between 1990 and 1994 and have been followed up over a period of 3 to 6 years. Materials and Methods: One hundred thirty-nine Calcitek dental implants were placed in 43 patients to support 14 maxillary and 30 mandibular overdentures. Standardized clinical review procedures were used. Results: At exposure all the implants were found to be osseointegrated. To date only 7 implants (5%) have completely failed and two associated overdentures (5%) have been lost. The cumulative survival rate of all implants over 6 years was 92%. However, if failure is defined by the loss of more than 4 mm of cervical bone, 33 implants could be classified as being in the process of failing. Using these figures, interval success rates as low as 82% were found by year 6, and the cumulative success rate would fall to 39%. Maxillary survival and success rates were siginficantly lower than mandibular rates, at 38% and 10%, respectively, by year 5. Conclusion: Failure rates were higher in the maxillary arch, in poor quality bone, in smokers, and where implants were opposed by a natural dentition or an implant-supported prosthesis. The results suggest that the cervical bone level adjacent to the Calcitek cylindrical hydro xyapatite-coated implant failed to establish a steady state, particularly in the maxillary arch. Doubts remain regarding the long-term prognosis of these cylindrical implants.