Evidence from randomized clinical trials of implant-retained overdentures is very limited at the present time. The aim of this study was to compare implant-retained mandibular overdentures and conventional complete dentures in a randomized controlled trial (RCT). Our a priori hypothesis was that implant-retained mandibular overdentures would be significantly better than conventional complete dentures. Edentulous patients (n = 118) were randomly allocated to either an Implant Group (n = 62) or a Denture Group (n = 56). Patients completed the Oral Health Impact Profile (OHIP) and a denture satisfaction scale pre-treatment and three months post-treatment. Upon completion of treatment, both groups reported improvement (p < 0.001, Wilcoxon Ranks Sum test) in oral-health-related quality of life and denture satisfaction. There were no significant post-treatment differences between the groups, but a treatment effect may be masked by application of "intention to treat" analysis. The OHIP change scores were significantly greater for patients receiving implants than for those who refused them.